Implement an ARCH_PACKAGES variable which defaults to BOARD but can be overridden...

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.

SVN-Revision: 23029