feat(cgpt): Statically allocate UUIDs for ROOT partitions.

Before we can switch from using device names in root= to partition table
UUIDs we need some values that will remain consistent across upgrades
since the partition table is not updated when filesystems are.
fix(build_image): Configure legacy bootloaders to set root by UUID.

Make use of the new partition UUIDs for ROOT-A and ROOT-B in the root=
kernel parameters provided by the legacy (non-kexec) bootloaders. This
makes all of our images bootable as-is without having to pass them
through image_to_vm.sh. :-D
Merge pull request #27 from marineam/rootfs

Rootfs by UUID