'bootonly.iso' components to create a smaller memory stick image.
This is useful for system recovery, where a full memstick.img image
is not necessarily needed (or wanted). In addition, it is possible to
do bootonly-style installation, where the base.txz, kernel.txz, etc.
are fetched from a remote source.
Provide backwards-compatible target (mini-memstick), to keep in sync
with the targets documented in release/Makefile.
Prompted by: wblock
MFC after: 1 week
X-MFC-after: 10.0-RELEASE
Sponsored by: The FreeBSD Foundation