Support the Discoverable Partitions Specification (DPS)

In the context of UKI support and "sealed" base images, the kernel command line is "fixed" in the UKI, thus we can not update it to set variables such as the root partition UUID or LUKS hints, etc.

To make this work, we should support the Discoverable Partitions Specification (DPS) and use fixed Partition Type UUIDs for the root at least, and maybe ESP & /boot filesystems (less important).