gradle: Allow bnd_build property to be empty
gradle: Allow bnd_build property to be empty

This change allows there to be no "master" project in the workspace.
Without a "master" project whose dependencies will seed the set of
projects to build, all non-private folders (names don't start with ".")
will be added to the set of gradle projects.

magnet/bnd,joansmith/bnd,xtracoder/bnd,joansmith/bnd,magnet/bnd,GEBIT/bnd,mcculls/bnd,mcculls/bnd,magnet/bnd,lostiniceland/bnd,lostiniceland/bnd,lostiniceland/bnd,psoreide/bnd,psoreide/bnd,GEBIT/bnd,xtracoder/bnd,psoreide/bnd,mcculls/bnd