boot_serial: Add optional img mgmt slot info feature #2080

Job	Run time
config (RELEASE 2,4,6 SIGNATURE)	9m 7s
config (RELEASE 2,4,6 SIGNATURE LOW)	13m 28s
config (RELEASE 2,4,6 SIGNATURE MEDIUM)	17m 22s
config (RELEASE 8,10 SIGNATURE)	6m 34s
config (RELEASE 8,10 SIGNATURE LOW)	9m 23s
config (RELEASE 8,10 SIGNATURE MEDIUM)	12m 2s
config (MINSIZEREL 2,4,6 SIGNATURE)	15m 11s
config (MINSIZEREL 2,4,6 SIGNATURE LOW)	19m 29s
config (MINSIZEREL 2,4,6 SIGNATURE MEDIUM)	24m 9s
config (MINSIZEREL 8,10 SIGNATURE)	10m 39s
config (MINSIZEREL 8,10 SIGNATURE LOW)	13m 48s
config (MINSIZEREL 8,10 SIGNATURE MEDIUM)	16m 22s
	2h 47m 34s

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