Raise the scope of the `IMAGES` config variable in the
`build/release/Makefile` to the `build/makelib/common.mk` file, and use
the `IMAGES` variable in the `images/Makefile` to allow users to build
only select images.

Examples of new use:
  make IMAGES='ceph ceph-toolbox' build
  make -C build/release IMAGES='ceph ceph-toolbox' publish

The 'images/image.mk' clean.build target was changed to use
`CLEAN_IMAGES` instead of `IMAGES`, as it was conflicting with use of
the parameter in image builds.

Added IMAGES option to top-level make's help text. The text for this
option exceeded 80 lines in the `Makefile` itself, so the help text was
refactored to eliminate `    @echo '<text>'`. The `Makefile` text should
now match exactly what is output to the user including line length.

Added IMAGES option to `INSTALL.md` file's "Build" section and also
moved some text around and added a few extra points of clarification
there to hopefully improve readability, especially for first-ti...