update/trees: further simplify crossgcc handling

arch no longer needs to be set, on multi-tree projects,
and it has been renamed to xarch

the new behaviour is: if xarch is set, treat it as a
list of crossgcc targets and go through the list. set
the first one as the target, for what lbmk builds, but
build all of the defined crossgccc targets

crossgcc_ada is now xlang, and defines which languages
to build, rather than whether to build gcc-gnat

Signed-off-by: Leah Rowe <leah@libreboot.org>

script/build/roms

@@ -21,7 +21,7 @@ kmapdir="config/grub/keymap"
 # target.cfg files have to specifically enable [a] payload(s)
 pv="payload_grub payload_grub_withseabios payload_seabios payload_memtest t"
 pv="${pv} payload_seabios_withgrub payload_seabios_grubonly payload_uboot memtest_bin"
-v="romdir cbrom initmode displaymode cbcfg targetdir tree arch keymaps"
+v="romdir cbrom initmode displaymode cbcfg targetdir tree keymaps"
 v="${v} grub_timeout ubdir board grub_scan_disk uboot_config"
 eval "$(setvars "n" ${pv})"
 eval "$(setvars "" ${v} boards _displaymode _payload _keyboard all targets)"
@@ -84,7 +84,6 @@ configure_target()
 	    && [ "${grub_scan_disk}" != "ahci" ] && \
 		grub_scan_disk="both"
 
-	[ -z "$arch" ] && err "$board: arch not defined"
 	[ -z "$tree" ] && err "$board: tree not defined"
 
 	[ "${payload_memtest}" != "y" ] && payload_memtest="n"