Make buildkernel's require a buildworld to be done first, because they

Make buildkernel's require a buildworld to be done first, because they
no longer munge the pathes to use native apps when buildworld tools aren't
available.

Buildkernel now tells you this and exits if it doesn't think you've done
a buildworld.

Add a new target, 'nativekernel', which just runs config and uses native
tools to build the kernel.  'nativekernel' and 'buildkernel' use the same
object directory but are mutually exclusive.  If you run one, then try to run
the other, it will wipe the directory and start over.