disk_util: Set priority and successful attributes in new images

Mark the initial copy of CoreOS as 'successful' and with a non-zero
priority. Required to boot with a stricter interpretation of the
partition selection scheme which ignores partitions that have a priority
of zero. The new grub implementation follows this rule and is what the
original ChromeOS spec used too.

For the sake of completeness if multiple partitions are configured in
the json file with this feature they will be prioritized in disk-order.
disk_util: Add support for computing verity hashes
scripts/oem/ami: fix bad array subscript

Broken in 8abceaa20e10ff5f3543a54e69b5accddfd5f7b9.
Merge pull request #353 from marineam/gptprio

disk_util: Set priority and successful attributes in new images
Merge pull request #354 from marineam/verity

Compute verity hashes of /usr filesystem
Merge pull request #355 from crawford/ami

scripts/oem/ami: fix bad array subscript