MFC r264027, r264028, r264029, r264030, r264046, r264073:

r264027:
  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.

r264028:
  Clean up trailing whitespace in release/Makefile.

r264029:
  Fix logic error.

r264030:
  If WITH_COMPRESSED_IMAGES is set, add the compressed images
  to the CLEANFILES list.

r264046:
  Use xz(1) instead of gzip(1) to compress release images
  when WITH_COMPRESSED_IMAGES is used.

r264073:
  Allow overriding xz(1) path.

Sponsored by:	The FreeBSD Foundation
MFC a modified version of r258588 (modification suggested by Mikolaj
Golub; tested stable both original and modified versions) that fixes
kernel panic when starting/stopping VIMAGE jails.

Discussed with:	rodrigc, Mikolaj Golub