New approach to disk/partition handling, disk_util

This renames cgpt.py to disk_util and reworks it with the goal of replacing disk related manipulation in:

    build_library/base_image_util.sh
    build_library/disk_layout_util.sh
    build_library/mount_gpt_util.sh
    mount_gpt_image.sh
    .../mount_image.sh
    .../umount_image.sh
    .../pack_partitions.sh
    .../unpack_partitions.sh

And parts of:

    build_library/vm_image_util.sh

The most interesting changes is the addition of `disk_util format`, `disk_util mount`, and `disk_util resize` commands which make filesystem types and mount locations defined in the partition layout config file to make it more practical to support building both the current and /usr layouts during the transition.