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