Add a hack that unsets REVISION when building a release.

Sadly what I'd really like to do here is set or unset REVISION
depending on whether a git tag or branch is being built.  This
isn't possible because the naming is calculated before any git
manipulations are done.

This is enough to allow x.y.z-RELEASE images to have a name that
doesn't include the git hash without remembering to set REVISION=""
before tagging.

This horrific hack could be extended to cover RCs but I couldn't
bring myself to do it.