build: submodule machinery now works also when no tag is reachable

build: submodule machinery now works also when no tag is reachable

The code in cfg.mk to detect when the git submodule was out of date
worked most of the time, but not when checked out in a certain way.
* cfg.mk: Extract submodule hash from command output and file,
and compare only that, since the format of the full line may vary.
Reported by Mike Burns, with some diagnosis by Daniel P Berrange.