Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Give mdconfig a couple of seconds to deconfigure in case debugging is being
printed to a serial console.

Reverse the order of args to diff to make it more logical.

Add script to run kernel based regression test and the reference vectors
for the current XML images.