Merge pull request #5 from walac/bugz/1087456

Bug 1087456
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.