Add a new release build variable, WITH_COMPRESSED_IMAGES.

When set to a non-empty value, the installation medium is
compressed as part of the 'install' target in the release/
directory.

Clean up trailing whitespace in release/Makefile.

Fix logic error.

If WITH_COMPRESSED_IMAGES is set, add the compressed images
to the CLEANFILES list.

Use xz(1) instead of gzip(1) to compress release images
when WITH_COMPRESSED_IMAGES is used.

Allow overriding xz(1) path.