bsdinstall: Guard against invalid branch/revision

I somehow ended up with a tree where make -V BRANCH and make -V REVISION
were broken, resulting in a FreeBSD-base.conf with a broken repo URL.
Check for BRANCH == RELEASE explicitly and emit a warning if BRANCH does
not match an expected case.

Reviewed by: Isaac Freund <ifreund@freebsdfoundation.org>
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D51528