Bug 900526, part 1: Allow inheritable moz.build variables, r=gps.
Bug 900526, part 2: Move backend.mk inclusion to config.mk instead of rules.mk, r=gps
Bug 900526, part 3: Define FINAL_TARGET and friends in moz.build, r=gps
Bug 900526, part 4: Migrate FINAL_TARGET and XPI_NAME to moz.build, r=gps
Bug 900526, part 5: Move DIST_SUBDIR to moz.build, r=gps