Undo some of the previously made changes to deal with cross build issues

Undo some of the previously made changes to deal with cross build issues
from 4.x and older versions of DFly.

sys.mk should *NOT* include bsd.init.mk.  That is way, way too early and
blows up .inc files, e.g. such as those used by sys/boot.

Rename .sub back to .inc in sys/boot (more on the way) so it will cross build
from 4.x.