Bug 1040009 - Run mozilla-central's configure from the objdir, not objdir/mozilla, r=Standard8 CLOSED TREE.

This bug moves the objdir from the current location of MOZ_OBJDIR/mozilla to
MOZ_OBJDIR, which fixes a series of longstanding bugs about mozconfigs not
working due to the presence of two distinct objdirs. A temporary workaround to
keep automation working is included.

An important implication of this change is that mozconfigs are now required for
comm-central: if they are not provided, Firefox, not Thunderbird, will be built.

--HG--
extra : rebase_source : 8a5f97dc6f65a9eeaa403b0886027f42158636ce