scripts/layerman: Run gc and prune dead branches

This would run git garbage collector and prune the
branches which has been deleted in upstream repo.

Signed-off-by: Khem Raj <raj.khem@gmail.com>
layers.txt: add meta-beagleboard layer

Signed-off-by: Koen Kooi <koen@dominion.thruhere.net>
layerman: add meta-networking, meta-webserver and meta-kde

Signed-off-by: Koen Kooi <koen@dominion.thruhere.net>
gitignore: ignore bitbake.lock

Its a build generated artifact

Signed-off-by: Khem Raj <raj.khem@gmail.com>