fix(prod_image_util): Do not install update test key anymore.

Leave this to the ebuild, no reason for it to be done here.
feat(disk_util): Add support for extracting partitions.

This will simplify the process of generating update payloads.
feat(build_image): Optionally generate update payload with prod images.
fix(build_image): Add group prefix to image output directories.
Merge pull request #237 from marineam/groups

Updates, keys, and groups