Compute verity hashes of /usr filesystem

No consideration is made for doing anything with the root hash yet, this is just a starting point to start tinkering with verity.

Depends on https://github.com/coreos/coreos-overlay/pull/961
Should merge after https://github.com/coreos/scripts/pull/353
Where does the hash end up being written? Is that right after the end of the filesystem but still within the partition?