bsd.progs.mk must pass META_XTRAS to gendirdeps.mk

The indirection used by bsd.progs.mk is setting META_XTRAS
means the value needs to be passed in the environment to
gendirdeps.mk, as any expansion before then will be empty.

Remove a now misleading comment from bsd.progs.mk
before it includes bsd.prog.mk

Update gendirdeps.mk to accommodate this.

Reviewed by: stevek