Bug 1085513 part1: Detect the mozconfig obj directory.

We cannot hardcode the object directory because it is configurable
inside the mozconfig file.

Parse the mozconfig and extract the object directory. If it is not
defined, defaults to "/home/worker/object-folder".
Bug 1085513 part2: use nightly mozconfig for b2g desktop builds.
Bug 1085513 part3: Remove unused mozconfig files.

Since we now use the gecko tree mozconfigs for building, the docker
mozconfigs are no longer necessary.
Merge pull request #6 from walac/bugz/1085513

Bug 1085513