udev-builtin-blkid: when we find a GPT partition marked as root disk on the same...

udev-builtin-blkid: when we find a GPT partition marked as root disk on the same disk as the ESP, expose a property on the udev device

This is preparation for a logic to automatically discover the root
partition to boot from if no partition has been configured explicitly.

This makes use of our newly defined GPT type GUIDs for our root disks:

 #define GPT_ROOT_X86    SD_ID128_MAKE(44,47,95,40,f2,97,41,b2,9a,f7,d1,31,d5,f0,45,8a)
 #define GPT_ROOT_X86_64 SD_ID128_MAKE(4f,68,bc,e3,e8,cd,4d,b1,96,e7,fb,ca,f9,84,b7,09)

We define differen GUIDs for different architectures to allow images
which finde the right root partition for the appropriate arch.