Add revision as a 4th component in the LAYER file format.

This makes the LAYER file much like the Angstrom setup-scripts
sources.txt file. It should also allow us to simplify the fetch script.
Use git parameters instead of descending into directories.

And a bit of cleanup too.
Use the name field when checking out repos.
Stop messing around with the build scripts directory in fetch.sh.
Get branch in manifest output.
Consolidate metadata collection / generation.
Move bitbake clone to same location as the meta layers.

Throw in some cleanup to get the oe-core scripts back on PATH.
Use soft assignment and append to variables.

This is in preparation for using auto.conf and site.conf.
Clean up ignores.
Include branch and HEAD in layer specification.