Add a 'mini-memstick.img' release target, which will use the
'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.

Remove extra target from 'memstick' in r257805.

Update release(7) to include 'mini-memstick' target.