Separate `root.patched` build target into `root.patched_common` & `root.patched_app`

[JedMeister]

During update & development it would speed up app rebuilds if the common and app specific build code (i.e. overlays and conf scripts) were applied separately via into their own build targets. E.g. rather than a single root.patched, apply them separately as root.patched_common (common build code) & root.patched_app (app specific) targets.

That way in many/most rebuilds you could just reapply the root.patched_app target (make root.patched_app), without needing to rebuild the whole root.patched target - which currently removes the whole root.patched deck and reapplies both common and app specific build code.