To ensure that I don't break the MK_ZFS=no case (currently working),
add that to the build, at least for i386 and amd64. Also, out of an
abundance of caution, clean single threaded, and then build in a
separate make. Otherwise, I hit a race where we build before objdir
has been created, which breaks at the moment.
Sponsored by: Netflix
One could run this from any directory, but it's designed to do
regression testing on sys/boot (it only tests on a subset of
architectures since all of them would take a lot longer and not help).
This will also ensure that future commits to sys/boot compile
everywhere.
Sponsored by: Netflix