hack(cgpt): use Root instead of the Kernel partitions

hack up the code a bit to set priorities on root partitions:

$ cgpt show -i 4 -P /dev/sda
1
$ cgpt show -i 5 -P /dev/sda
2

$ cgpt prioritize -i 4 /dev/sda
$ cgpt show -i 4 -P /dev/sda
2
chore(cgpt): refactor IsWholeDisk

make IsWholeDisk available as a library function. Move it into common.
feat(cgpt/cpgt_next): add a new cgpt command

cgpt next will return the partition uuid of the next partition that
should be booted. This can be used in an initrd to figure out what root
partition to use.

It handles decrementing tries so this is a one shot command.
fix(cgpt/cmd_next): don't' require a type arg

We don't have the type logic yet. Just don't worry about this for now.
chore(cgpt): remove TODO message

This TODO talked about code that is now gone. Remove it.
chore(cgpt/*next.c): add copyright headers

Add copyright headers to these files for CoreOS. There are clearly some
pieces of copied code but some original stuff too so leave Chromium on
too.
feat(cgpt/cmd_next): add a long form explanation

add notes on what this command does and how it works.
Merge pull request #2 from philips/add-next-cmd

Add next cmd