Move backup related stuff to its own Makefile.

This will prevent Makefile errors from affecting backups. This happens when
different branches include Makefile helpers other branches don't have. This
should be more reliable.
auto merge of #1294 : metajack/servo/isolate-backup, r=larsbergstrom

This will prevent Makefile errors from affecting backups. This happens when
different branches include Makefile helpers other branches don't have. This
should be more reliable.