Make: missing dependencies / build not reproducible

[tq-niebelm]

When changing the SM configuration a rebuild of the files under configs/$(CONFIG) is not triggered. For instance if the configs/$(CONFIG) exists and going some commits backward for testing.

The cfg rule should depend on the actual used configuration. Ideally this should not only be a a simply make dependency but rely on a hash of the config file used to generate the configuration, too. This should ensure if the configuration file changed that a rebuild takes place in any case.