MFC r330077, r330248

r330077:
Move EFI up to common makefile. There's no need for all these .if's
based on architecture.

Sponsored by: Netflix

r330248:
Create LOADER_UBOOT, and LOADER_OFW. Move these options out of
Makefile.${MACHINE_ARCH} and remove the now-empty files. Generate the
*32 directories on the necessary architectures (well, currently only
amd64) on the fly. Remove LOADER_EFI variable and co-locate it with
EFI.