Bug 931017, part 1: include backend.mk from config.mk instead of rules.mk, r=Standard8

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extra : rebase_source : 40dd2389523b7c55ebcd8e81713d64d31ad8cf5c
Bug 931017, part 2: Move FINAL_TARGET and friends to moz.build, r=Standard8
CLOSED TREE changes to SeaMonkey in mass-migration.

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extra : rebase_source : 0845dafeeefb96ac331bc34c4f4133cac326d628
extra : amend_source : d83d817d7d9c2c7349ff1d67c3e0d1d010e7c83f