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.

Differential Review: https://reviews.freebsd.org/D14546