Use .mozconfig files in tor-browser repo for rbm builds

Right now, if we need to change .mozconfig files, we need to change them both in the tor-browser repository and in tor-browser-build (see the respective note in README.HACKING).

We should find a way to leave the .mozconfig files in the tor-browser repository for local builds outside of rbm (and only change them there when writing tor-browser patches) but use them nevertheless for rbm-based builds (probably with some modifications if needed).