release/Makefile:
  Connect the virtual machine image build to the release
  target if WITH_VMIMAGES is set to a non-empty value.

release/release.sh:
  Add WITH_VMIMAGES to RELEASE_RMAKEFLAGS.

release/release.conf.sample:
  Add commented entries for tuning the release build if the
  WITH_VMIMAGES make(1) environment variable is set to
  a non-empty value.

Sponsored by:	The FreeBSD Foundation