Make sure R.stamp is always created

When an app has a resource directory, but the
directory contain no resources, R.stamp is not created.

This leads to that such apps are always rebuilt,
even when there are no changes.

This patch changes so that an empty R.stamp is
created even if there are no resources.
This leads to that affected apps are not rebuilt
unless changed.

Change-Id: Ia63e5b1913a4456402700b615ba7f9d56d2da852