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
To make room at between the end of the filesystem and the end of the partition for the verity hashes. Alternatively a new partition could be added but keeping the fs and hash tree as a single unit avoids adding an extra step to the update process.