This is mostly to allow using MAKEOBJDIRPREFIX in src-env.conf on stable where
src.sys.obj.mk is not going to be MFC'd.  It is still valid on head but
effectively a NOP due to MAKEOBJDIRPREFIX being handled differently in
src.sys.obj.mk.

Reported by:	eadler
MFC after:	1 week
Sponsored by:	Dell EMC