build: help text includes available device profiles

[pktpls]

The old script had a -l option which listed the available device profiles for the selected target.

For the new script we let the usage help text cover this. The script arguments are checked and the help text printed very early. To know which device profiles are available, we need the imagebuilder to be already downloaded and extracted, so in this case we print the help text a little later.

Example:

> build/build.sh snapshot ipq40xx/generic
[... debugging output ...]
usage: build/build.sh <version> <target> <profile> [<destination>]

versions and release branches:
  snapshot => snapshot
  1.4.x => 1.4.0-snapshot
  1.3.x => 1.3.0-snapshot
  1.2.x => 1.2.3-snapshot
  testbuildbot => testbuildbot
  anything else => snapshot

target names:
  apm821xx/nand  apm821xx/sata  armsr/armv7  armsr/armv8  at91/sam9x  at91/sama5  at91/sama7  ath79/generic  ath79/mikrotik  ath79/nand  ath79/tiny  bcm27xx/bcm2708  bcm27xx/bcm2709  bcm27xx/bcm2710  bcm27xx/bcm2711  bcm27xx/bcm2712  bcm47xx/generic  bcm47xx/legacy  bcm47xx/mips74k  bcm4908/generic  bcm53xx/generic  bmips/bcm6318  bmips/bcm63268  bmips/bcm6328  bmips/bcm6358  bmips/bcm6362  bmips/bcm6368  d1/generic  gemini/generic  imx/cortexa53  imx/cortexa7  imx/cortexa9  ipq40xx/chromium  ipq40xx/generic  ipq40xx/mikrotik  ipq806x/chromium  ipq806x/generic  ixp4xx/generic  kirkwood/generic  lantiq/ase  lantiq/xrx200  lantiq/xway  lantiq/xway_legacy  layerscape/armv7  layerscape/armv8_64b  loongarch64/generic  malta/be  mediatek/filogic  mediatek/mt7622  mediatek/mt7623  mediatek/mt7629  mpc85xx/p1010  mpc85xx/p1020  mpc85xx/p2020  mvebu/cortexa53  mvebu/cortexa72  mvebu/cortexa9  mxs/generic  octeon/generic  octeontx/generic  pistachio/generic  qualcommax/ipq807x  ramips/mt7620  ramips/mt7621  ramips/mt76x8  ramips/rt288x  ramips/rt305x  ramips/rt3883  realtek/rtl838x  realtek/rtl839x  realtek/rtl930x  realtek/rtl931x  rockchip/armv8  sifiveu/generic  starfive/generic  sunxi/cortexa53  sunxi/cortexa7  sunxi/cortexa8  tegra/generic  x86/64  x86/generic  x86/geode  x86/legacy  zynq/generic

profile names:
  all  8dev_habanero-dvk  8dev_jalapeno  alfa-network_ap120c-ac  aruba_ap-303  aruba_ap-303h  aruba_ap-365  asus_map-ac2200  asus_rt-ac42u  asus_rt-ac58u  avm_fritzbox-4040  avm_fritzbox-7530  avm_fritzrepeater-1200  avm_fritzrepeater-3000  buffalo_wtr-m2133hp  cellc_rtl30vw  compex_wpj428  devolo_magic-2-wifi-next  dlink_dap-2610  edgecore_ecw5211  engenius_eap1300  extreme-networks_ws-ap3915i  extreme-networks_ws-ap391x  ezviz_cs-w3-wd1200g-eup  glinet_gl-a1300  glinet_gl-ap1300  glinet_gl-b1300  glinet_gl-b2200  linksys_ea6350v3  linksys_ea8300  linksys_mr8300  linksys_whw03  linksys_whw03v2  linksys_whw01  luma_wrtq-329acn  meraki_mr33  meraki_mr74  mobipromo_cm520-79f  netgear_ex6100v2  netgear_ex6150v2  netgear_lbr20  netgear_rbr40  netgear_rbs40  netgear_rbr50  netgear_rbs50  netgear_srr60  netgear_srs60  netgear_wac510  openmesh_a42  openmesh_a62  p2w_r619ac-64m  p2w_r619ac-128m  pakedge_wr-1  plasmacloud_pa1200  plasmacloud_pa2200  qxwlan_e2600ac-c1  qxwlan_e2600ac-c2  sony_ncp-hg100-cellular  teltonika_rutx50  wallys_dr40x9  yyets_le1  zte_mf18a  zte_mf282plus  zte_mf286d  zte_mf287plus  zte_mf287  zte_mf287pro  zte_mf289f  zyxel_nbg6617

destination:
  path to a writable directory where image files will end up.
  default: ./out

FALTER_VARIANT env variable:
  chooses the packageset variant. (deprecated)
  default: tunneldigger

FALTER_FEED env variable:
  customizes the falter package feed.
  default: https://firmware.berlin.freifunk.net/feed/<branch>/packages/<arch>/falter
  example: file:///tmp/falter-packages/out/snapshot/x86/64/falter