This means that the kernel can be totally self contained now and is not
dependent on the last buildworld to update /usr/share/mk. This might
also make it easier to build 5.x kernels on 4.0 boxes etc, assuming
gensetdefs and config(8) are updated.
needed for ages, but keeps getting cut/pasted into new Makefiles.
(Once apon a time it was used to activate mount arguments in
<sys/mount.h>, but that was killed with extreme prejudice long ago)
