Bug 1042432 - Change how UPLOAD_EXTRA_FILES is passed down from mozconfig.cache to the build system. r=mshal
Bug 1053069 - Make mozbuild.util.ReadOnlyDefaultDict raise an exception on update() and del. r=gps
Bug 1053070 - Output a nicer error when reassigning a variable in moz.build. r=gps
Bug 1053071 - Simplify EXTERNAL_SOURCE_DIR handling. r=gps

As handled in configure.in, EXTERNAL_SOURCE_DIR is a single directory for
external sources, and more specifically, comm-central. There is no need
to handle it as a list in the moz.build emitter.

At the same time, avoid checking the external sources for every moz.build,
and bind it to a config environment.
Bug 1053072 - Cleanup metadata in the moz.build reader. r=gps

- parent and var were used for TOOL_DIRS/TEST_TOOL_DIRS, which were removed in
  bug 1043802.
- tier metadata was not really used, and tiers are going to go away soon
   anyways.