Taking care of layer dependencies

The layers added in LAYERDEPENDS_layername will be
added to bblayers.conf. And sourcing of post-setup-environment
script is also done, if file exists, to make additional changes in local.conf

Signed-off-by: Sujith H <Sujith_Haridasan@mentor.com>
Merge pull request #69 from sharidas/setup-env-changes

Taking care of dependecny layers for qt5-mel and atp