Check early that frozen modules point at tag

[jepler]

Late during a build you can get a failure like

subprocess.CalledProcessError: Command 'git describe --tags --exact-match' returned non-zero exit status 128.

let's add a check early (before any board builds are started) that all frozen modules are pointing at a tag, so that hundreds of board builds can be avoided when this kind of problem exists. Prompted by problems encountered by a contributor in #7135