fix(build_image): Set read-only hack on correct filesystem.

Check if the disk layout is a /usr layout and if so hack the USR-A
partition, not whatever is mounted to /. Also use the new functionality
in disk_util for this as it can look up partitions by label.
Merge pull request #149 from marineam/usr-fixes

fix(build_image): Set read-only hack on correct filesystem.