Add meta-webos-backports layer

* bblayers-conf.in assumes that meta-webos-backports checkout is
  always the same as meta-webos checkout + "-backports" suffix
* mcf will always force meta-webos-backports submodule, because
  we'll have .bbappends in meta-webos which require meta-webos-backports
  so we always need to enable both
Merge pull request #39 from shr-project/backports-master

Add meta-webos-backports layer