gpt: add a new generic GUID type

In order to do anything with partition GUIDs they need to be stored in a
proper structure like the partition type GUIDs. Additionally add an
initializer macro to simplify defining both GUID types.
gpt: split out checksum recomputation

For basic data modifications the full repair function is overkill.
gpt: new gptprio.next command for selecting priority based partitions

Basic usage would look something like this:

    gptprio.next -d usr_dev -u usr_uuid
    linuxefi ($usr_dev)/boot/vmlinuz mount.usr=PARTUUID=$usr_uuid

After booting the system should set the 'successful' bit on the
partition that was used.
Merge pull request #5 from marineam/gptprio

Initial implementaiton of the gptprio 'next' command for grub.