Clean up P6 SDK paths

[mspang]

Problem

P6 SDK sources are specified relative to ${chip_root} which prevents
alternate repository layouts.

Change overview

Introduce a new variable p6_sdk_root for the actual root of the SDK
repository and use this to specify SDK paths.

This avoids forcing locating the SDK submodule at a particular location
in the Matter SDK directory, as projects may want to use a different
layout.

Testing

CI

[github-actions]

PR #20624: Size comparison from 761a9e9 to 39351ce

Increases (5 builds for bl602, cc13x2_26x2, cyw30739, telink)

platform	target	config	section	761a9e9	39351ce	change	% change
bl602	lighting-app	bl602+rpc	(read/write)	1442810	1442818	8	0.0
			.text	1090272	1090276	4	0.0
cc13x2_26x2	lock-mtd	LP_CC2652R7	(read only)	618583	618591	8	0.0
			.text	541936	541944	8	0.0
	pump-app	LP_CC2652R7	(read/write)	164032	164040	8	0.0
cyw30739	ota-requestor-no-progress-logging	cyw930739m2evb_01	(read/write)	582766	582774	8	0.0
			.app_xip_area	462368	462376	8	0.0
telink	light-switch-app	tlsr9518adk80d	text	565676	565678	2	0.0

Decreases (1 build for cc13x2_26x2)

platform	target	config	section	761a9e9	39351ce	change	% change
cc13x2_26x2	pump-app	LP_CC2652R7	(read only)	678183	678175	-8	-0.0
			.text	589196	589188	-8	-0.0

Full report (43 builds for bl602, cc13x2_26x2, cyw30739, efr32, esp32, k32w, linux, mbed, nrfconnect, p6, telink)

platform	target	config	section	761a9e9	39351ce	change	% change
bl602	lighting-app	bl602	(read/write)	1397386	1397386	0	0.0
			.bss	116978	116978	0	0.0
			.data	4480	4480	0	0.0
			.text	1058588	1058588	0	0.0
		bl602+rpc	(read/write)	1442810	1442818	8	0.0
			.bss	124418	124418	0	0.0
			.data	4600	4600	0	0.0
			.text	1090272	1090276	4	0.0
cc13x2_26x2	all-clusters-app	LP_CC2652R7	(read only)	666275	666275	0	0.0
			(read/write)	184948	184948	0	0.0
			.bss	74116	74116	0	0.0
			.data	3356	3356	0	0.0
			.rodata	88139	88139	0	0.0
			.text	577820	577820	0	0.0
	all-clusters-minimal-app	LP_CC2652R7	(read only)	632083	632083	0	0.0
			(read/write)	157684	157684	0	0.0
			.bss	73412	73412	0	0.0
			.data	3356	3356	0	0.0
			.rodata	77379	77379	0	0.0
			.text	554380	554380	0	0.0
	lock-ftd	LP_CC2652R7	(read only)	669183	669183	0	0.0
			(read/write)	172184	172184	0	0.0
			.bss	71148	71148	0	0.0
			.data	3280	3280	0	0.0
			.rodata	76279	76279	0	0.0
			.text	592424	592424	0	0.0
	lock-mtd	LP_CC2652R7	(read only)	618583	618591	8	0.0
			(read/write)	144264	144264	0	0.0
			.bss	66868	66868	0	0.0
			.data	3280	3280	0	0.0
			.rodata	76159	76159	0	0.0
			.text	541936	541944	8	0.0
	pump-app	LP_CC2652R7	(read only)	678183	678175	-8	-0.0
			(read/write)	164032	164040	8	0.0
			.bss	71228	71228	0	0.0
			.data	3280	3280	0	0.0
			.rodata	88503	88503	0	0.0
			.text	589196	589188	-8	-0.0
	pump-controller-app	LP_CC2652R7	(read only)	664007	664007	0	0.0
			(read/write)	178328	178328	0	0.0
			.bss	71348	71348	0	0.0
			.data	3276	3276	0	0.0
			.rodata	84367	84367	0	0.0
			.text	579160	579160	0	0.0
	shell	LP_CC2652R7	(read only)	658774	658774	0	0.0
			(read/write)	187952	187952	0	0.0
			.bss	76420	76420	0	0.0
			.data	3360	3360	0	0.0
			.rodata	84918	84918	0	0.0
			.text	573540	573540	0	0.0
cyw30739	light	cyw930739m2evb_01	(read/write)	579622	579622	0	0.0
			.app_xip_area	458376	458376	0	0.0
			.bss	64184	64184	0	0.0
			.data	716	716	0	0.0
			.rodata	0	0	0	0.0
			.text	112	112	0	0.0
	lock	cyw930739m2evb_01	(read/write)	585574	585574	0	0.0
			.app_xip_area	459600	459600	0	0.0
			.bss	68912	68912	0	0.0
			.data	720	720	0	0.0
			.rodata	0	0	0	0.0
			.text	112	112	0	0.0
	ota-requestor-no-progress-logging	cyw930739m2evb_01	(read/write)	582766	582774	8	0.0
			.app_xip_area	462368	462376	8	0.0
			.bss	63392	63392	0	0.0
			.data	660	660	0	0.0
			.rodata	0	0	0	0.0
			.text	112	112	0	0.0
efr32	lighting-app	BRD4161A	(read/write)	1081436	1081436	0	0.0
			.bss	132996	132996	0	0.0
			.data	2048	2048	0	0.0
			.text	946372	946372	0	0.0
		BRD4161A+rpc	(read/write)	1135764	1135764	0	0.0
			.bss	149676	149676	0	0.0
			.data	2260	2260	0	0.0
			.text	983808	983808	0	0.0
		BRD4161A+rs911x	(read/write)	947924	947924	0	0.0
			.bss	140768	140768	0	0.0
			.data	2048	2048	0	0.0
			.text	805088	805088	0	0.0
	lock-app	BRD4161A+wf200	(read/write)	1128848	1128848	0	0.0
			.bss	144184	144184	0	0.0
			.data	2060	2060	0	0.0
			.text	982580	982580	0	0.0
	window-app	BRD4161A	(read/write)	1075244	1075244	0	0.0
			.bss	134468	134468	0	0.0
			.data	2076	2076	0	0.0
			.text	938676	938676	0	0.0
esp32	all-clusters-app	c3devkit	(read only)	1020100	1020100	0	0.0
			(read/write)	1485642	1485642	0	0.0
			.dram0.bss	70080	70080	0	0.0
			.dram0.data	14600	14600	0	0.0
			.flash.rodata	215528	215528	0	0.0
			.flash.text	1020100	1020100	0	0.0
			.iram0.text	62902	62902	0	0.0
		m5stack	(read only)	1073967	1073967	0	0.0
			(read/write)	487712	487712	0	0.0
			.dram0.bss	75600	75600	0	0.0
			.dram0.data	34144	34144	0	0.0
			.flash.rodata	245972	245972	0	0.0
			.flash.text	1068583	1068583	0	0.0
			.iram0.text	123267	123267	0	0.0
k32w	light	k32w061+release	(read/write)	658832	658832	0	0.0
			.bss	69516	69516	0	0.0
			.data	1992	1992	0	0.0
			.text	581524	581524	0	0.0
	lock	k32w061+release	(read/write)	685684	685684	0	0.0
			.bss	69980	69980	0	0.0
			.data	2004	2004	0	0.0
			.text	607900	607900	0	0.0
linux	all-clusters-app	debug	(read only)	2960825	2960825	0	0.0
			(read/write)	154752	154752	0	0.0
			.bss	61536	61536	0	0.0
			.data	2048	2048	0	0.0
			.data.rel.ro	84968	84968	0	0.0
			.dynamic	608	608	0	0.0
			.got	4536	4536	0	0.0
			.init	27	27	0	0.0
			.init_array	1048	1048	0	0.0
			.rodata	263613	263613	0	0.0
			.text	2520002	2520002	0	0.0
	all-clusters-minimal-app	debug	(read only)	2813401	2813401	0	0.0
			(read/write)	146688	146688	0	0.0
			.bss	60864	60864	0	0.0
			.data	2048	2048	0	0.0
			.data.rel.ro	77608	77608	0	0.0
			.dynamic	608	608	0	0.0
			.got	4488	4488	0	0.0
			.init	27	27	0	0.0
			.init_array	1048	1048	0	0.0
			.rodata	265341	265341	0	0.0
			.text	2373026	2373026	0	0.0
	bridge-app	debug+rpc	(read only)	2315449	2315449	0	0.0
			(read/write)	125504	125504	0	0.0
			.bss	48928	48928	0	0.0
			.data	3824	3824	0	0.0
			.data.rel.ro	66984	66984	0	0.0
			.dynamic	608	608	0	0.0
			.got	4392	4392	0	0.0
			.init	27	27	0	0.0
			.init_array	728	728	0	0.0
			.rodata	198016	198016	0	0.0
			.text	1955698	1955698	0	0.0
	chip-tool	debug	(read only)	10345025	10345025	0	0.0
			(read/write)	622240	622240	0	0.0
			.bss	24728	24728	0	0.0
			.data	3234	3234	0	0.0
			.data.rel.ro	587888	587888	0	0.0
			.dynamic	608	608	0	0.0
			.got	5096	5096	0	0.0
			.init	27	27	0	0.0
			.init_array	640	640	0	0.0
			.rodata	515861	515861	0	0.0
			.text	8397476	8397476	0	0.0
	chip-tool-no-interactive-ipv6only	arm64	(read only)	10031716	10031716	0	0.0
			(read/write)	684529	684529	0	0.0
			.bss	42609	42609	0	0.0
			.data	1152	1152	0	0.0
			.data.rel.ro	623432	623432	0	0.0
			.dynamic	528	528	0	0.0
			.got	13520	13520	0	0.0
			.init	24	24	0	0.0
			.init_array	192	192	0	0.0
			.rodata	478260	478260	0	0.0
			.text	7992788	7992788	0	0.0
	lighting-app	debug+rpc	(read only)	2551193	2551193	0	0.0
			(read/write)	129528	129528	0	0.0
			.bss	49440	49440	0	0.0
			.data	2096	2096	0	0.0
			.data.rel.ro	72136	72136	0	0.0
			.dynamic	608	608	0	0.0
			.got	4392	4392	0	0.0
			.init	27	27	0	0.0
			.init_array	816	816	0	0.0
			.rodata	213704	213704	0	0.0
			.text	2167522	2167522	0	0.0
	lock-app	debug	(read only)	2515913	2515913	0	0.0
			(read/write)	124512	124512	0	0.0
			.bss	47840	47840	0	0.0
			.data	1712	1712	0	0.0
			.data.rel.ro	69096	69096	0	0.0
			.dynamic	608	608	0	0.0
			.got	4424	4424	0	0.0
			.init	27	27	0	0.0
			.init_array	792	792	0	0.0
			.rodata	228744	228744	0	0.0
			.text	2122002	2122002	0	0.0
	ota-provider-app	debug	(read only)	2322305	2322305	0	0.0
			(read/write)	118312	118312	0	0.0
			.bss	47488	47488	0	0.0
			.data	1944	1944	0	0.0
			.data.rel.ro	63096	63096	0	0.0
			.dynamic	608	608	0	0.0
			.got	4488	4488	0	0.0
			.init	27	27	0	0.0
			.init_array	672	672	0	0.0
			.rodata	203512	203512	0	0.0
			.text	1956018	1956018	0	0.0
	ota-requestor-app	debug	(read only)	2439361	2439361	0	0.0
			(read/write)	125216	125216	0	0.0
			.bss	49856	49856	0	0.0
			.data	2232	2232	0	0.0
			.data.rel.ro	67288	67288	0	0.0
			.dynamic	608	608	0	0.0
			.got	4480	4480	0	0.0
			.init	27	27	0	0.0
			.init_array	728	728	0	0.0
			.rodata	207296	207296	0	0.0
			.text	2060914	2060914	0	0.0
	shell	debug	(read only)	2551169	2551169	0	0.0
			(read/write)	141104	141104	0	0.0
			.bss	57448	57448	0	0.0
			.data	1264	1264	0	0.0
			.data.rel.ro	76688	76688	0	0.0
			.dynamic	608	608	0	0.0
			.got	4136	4136	0	0.0
			.init	27	27	0	0.0
			.init_array	928	928	0	0.0
			.rodata	227762	227762	0	0.0
			.text	2166306	2166306	0	0.0
	thermostat-no-ble	arm64	(read only)	2595316	2595316	0	0.0
			(read/write)	158289	158289	0	0.0
			.bss	65249	65249	0	0.0
			.data	1704	1704	0	0.0
			.data.rel.ro	83240	83240	0	0.0
			.dynamic	528	528	0	0.0
			.got	5072	5072	0	0.0
			.init	24	24	0	0.0
			.init_array	400	400	0	0.0
			.rodata	165476	165476	0	0.0
			.text	2190064	2190064	0	0.0
	tv-app	debug	(read only)	3102225	3102225	0	0.0
			(read/write)	257704	257704	0	0.0
			.bss	167016	167016	0	0.0
			.data	4848	4848	0	0.0
			.data.rel.ro	79392	79392	0	0.0
			.dynamic	608	608	0	0.0
			.got	4848	4848	0	0.0
			.init	27	27	0	0.0
			.init_array	952	952	0	0.0
			.rodata	249024	249024	0	0.0
			.text	2665298	2665298	0	0.0
	tv-casting-app	debug	(read only)	5577785	5577785	0	0.0
			(read/write)	161968	161968	0	0.0
			.bss	50248	50248	0	0.0
			.data	2416	2416	0	0.0
			.data.rel.ro	103048	103048	0	0.0
			.dynamic	608	608	0	0.0
			.got	4744	4744	0	0.0
			.init	27	27	0	0.0
			.init_array	864	864	0	0.0
			.rodata	343209	343209	0	0.0
			.text	4956626	4956626	0	0.0
mbed	lock-app	CY8CPROTO_062_4343W+release	(read only)	6224	6224	0	0.0
			(read/write)	2448176	2448176	0	0.0
			.bss	213940	213940	0	0.0
			.data	5872	5872	0	0.0
			.text	1410820	1410820	0	0.0
nrfconnect	all-clusters-app	nrf52840dk_nrf52840	(read/write)	1175175	1175175	0	0.0
			bss	142900	142900	0	0.0
			rodata	141888	141888	0	0.0
			text	811492	811492	0	0.0
	all-clusters-minimal-app	nrf52840dk_nrf52840	(read/write)	1155367	1155367	0	0.0
			bss	142136	142136	0	0.0
			rodata	133416	133416	0	0.0
			text	800936	800936	0	0.0
p6	all-clusters-app	default	(read/write)	2566144	2566144	0	0.0
			.bss	149120	149120	0	0.0
			.data	2776	2776	0	0.0
			.text	1524408	1524408	0	0.0
	all-clusters-minimal-app	default	(read/write)	2511440	2511440	0	0.0
			.bss	148400	148400	0	0.0
			.data	2776	2776	0	0.0
			.text	1469704	1469704	0	0.0
	light-app	default	(read/write)	2441368	2441368	0	0.0
			.bss	140456	140456	0	0.0
			.data	2592	2592	0	0.0
			.text	1399632	1399632	0	0.0
	lock-app	default	(read/write)	2468520	2468520	0	0.0
			.bss	140304	140304	0	0.0
			.data	2600	2600	0	0.0
			.text	1426784	1426784	0	0.0
telink	light-switch-app	tlsr9518adk80d	(read/write)	797284	797284	0	0.0
			bss	70576	70576	0	0.0
			noinit	40416	40416	0	0.0
			text	565676	565678	2	0.0
	lighting-app	tlsr9518adk80d	(read/write)	817116	817116	0	0.0
			bss	71420	71420	0	0.0
			noinit	40416	40416	0	0.0
			text	582002	582002	0	0.0