Add support for multiple rbm.conf files

As the rbm.conf in tor-browser-build has become quite big, I think it would be nice to be able split rbm.conf into multiple files.

For other projects reusing some parts of tor-browser-build (for example ricochet-refresh-build, or namecoin), it would make it easier to reuse some part of rbm.conf without taking the entire file.

/cc @richard @JeremyRand