ostree-build-yocto: Ensure we point bblayers.conf at our checkout

Port from gnome-ostree, see
https://git.gnome.org/browse/gnome-ostree/commit/?id=48994dbdc45b99941fe001b971b382f3497b7652
ostree-build-yocto: Read LAYERS file

Port from gnome-ostree, see
https://git.gnome.org/browse/gnome-ostree/commit/?id=7218965fded504c20d45c54b154266e3837f7628
build: Don't force rebuild yocto if local:

Building yocto takes time and it's easy enough to clear the cache
if we want.
buildsystems: Delete static libraries

Delete all .la and .a files.
buildapi: Follow sbindir merge with bindir

We now install all binaries into usr/bin.
build, vcs: Fix using local: for patches

Use patches from local: instead of git: when needed.