Bug 1035599: Merge the mozilla-central and comm-central objdir, r=gps

This change merges mozilla-central and comm-central into having a single
topobjdir file but retaining two topsrcdirs (and two build systems). This state
is hopefully only the first part of a series of changes that eliminate the
comm-central build system partial clone completely.

--HG--
extra : rebase_source : 02aa2c4551df405d9783ac85cc41fe90b67bf057
extra : amend_source : d3cc677d59603648165bf65afa28413f2c40b2fd
Bug 1035599: Fix branding directory location for comm-central, r=glandium