Loader: support booting OS from memory disk (MD)

Until now, the boot image can be embedded into the loader with
/sys/tools/embed_mfs.sh, and memory disk (MD) is already supported
in loader source. But due to memory disk (MD) driver isn't registered
to the loader yet, the boot image can't be boot from embedded memory
disk.

Reviewed by: dab, tsoome
MFC after: 1 week
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D29512

(cherry picked from commit 5984246f9626fbc3d356ee2d3b3cd159f6d0a7c2)