MFC r257805, r257806, r257807:

MFC r257805, r257806, r257807:

r257805:
  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.

r257806:
  Remove extra target from 'memstick' in r257805.

r257807:
  Update release(7) to include 'mini-memstick' target.

Sponsored by:	The FreeBSD Foundation

git-svn-id: svn://svn.freebsd.org/base/stable/9@258312 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f