bug#35856: Unable to boot on iMac, likely because of UUIDs

[Francisco Gómez]

When I have inserted a device recorded with a GuixSD image in my Late 
2009 iMac (EFI, not UEFI) during boot, the computer gets stuck in a 
loop and doesn't respond. I believe this is due to a lack of, or 
duplication, of UUIDs (not to be confused with PARTUUIDs) on some 
partitions, as manually assigning them allows my computer to properly 
detect the attached devices and boot from the GuixSD one (although not 
successfully).

> udisksctl loop-setup -rf guix-system-install-1.0.1.x86_64-linux.iso
Mapped file guix-system-install-1.0.1.x86_64-linux.iso as /dev/loop0.

> sudo blkid -o list | grep ^/dev/loop0 | cut -d" " -f1 | sort
/dev/loop0
/dev/loop0p1
/dev/loop0p2
/dev/loop0p3
/dev/loop0p4

> sudo blkid -s UUID | grep ^/dev/loop0
/dev/loop0: UUID="1970-01-01-19-49-46-83"
/dev/loop0p2: UUID="C920-B8AE"