Code review comment for lp:~pundiramit/linaro-android-build-tools/access_private_manifests

This generally looks good. I think only one thing is missing: what happens when someone puts the SOURCE_OVERLAY_OPTIONAL in the build configuration on android-build?

Since the config file is generated on the build with the script

  http://bazaar.launchpad.net/~linaro-infrastructure/linaro-android-build-tools/trunk/view/head:/node/prepare_build_config.py

based on the data passed in from android-build.linaro.org build config, we should make sure that the SOURCE_OVERLAY_OPTIONAL triggers an error there.