[buildroot] Implement an ARCH_PACKAGES variable which defaults to BOARD but...

[buildroot] Implement an ARCH_PACKAGES variable which defaults to BOARD but can be overridden on a per-subtarget basis.
This is intended for targets such as adm5120 and malta which support both big and little endianess and will replace
the target specific exceptions in package-defaults and Image Builder.


git-svn-id: svn://svn.openwrt.org/openwrt/trunk@23029 3c298f89-4303-0410-b956-a3cf2f4a3e73