fetch.sh: Get rid of submodule support.

This hasn't been tested or used in a long time. Probably doesn't
work. Best to just be rid of it.
fetch.sh: Restructure LAYERS and fetch.sh accordingly.

The goal here is to make the LAYERS file capable of being a sort of
manifest that can be used to reproduce builds. This means allowing
for the use of an object hash as the desired state of a repo. Next
step is generating a LAYERS file from the current state of the build
directory.
fetch.sh: If oe-build-scripts is listed in manifest we error out of our local copy disagrees with the manifest.
manifest_gen.sh: Add new script to generate a manifest (LAYERS file) describing the state of git repos.