fix(disk_util): Fix layout inheritance in config verification.
fix(disk_util): Sort mount order instead of special casing /

Now that /usr is a mount point we need to make sure it gets mounted
before /usr/share/oem. Simply sorting by path length works well enough.
fix(build_library): remove references to chromeos-common.sh

Nothing from chromeos-common.sh is needed for image building now. Also
kill off build_common.sh which was just a weird way of sourcing
common.sh. The two piddly functions it provided fit better in
build_image_util.sh