MFC r257805, r257806, r257807, r257942:

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' that I forgot to remove before
  previous commit.

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

r257942:
  Fix a few style nits.

Sponsored by:	The FreeBSD Foundation