gpt-auto-generator: automatically find the root disk of the system

gpt-auto-generator: automatically find the root disk of the system

When run in an initrd and no root= argument is set (or is set to
root=gpt-auto) we will automatically look for the root partition on the
same disk the EFI ESP is located on.

Since we look for swap, /home and /srv on the disk the root partition is
located on, we hence have a fully discoverable chain:

    Firmware discovers the EFI ESP partition → the initrd discovers the
    root partition → the host OS discovers swap, /home, and /srv.

Note that this requires an EFI boot loader that sets the
LoaderDevicePartUUID EFI variable, such as Gummiboot.