Currently we run grub-mkconfig then go and edit the generated output
in /boot/grub/grub.cfg to override the "root=" arguments to our label.
If another element like project-config's
finalise.d/99-fix-grub-timeout then goes and re-runs grub2-install, it
overwrites these changes.
Grub has GRUB_DEVICE which should override the root device. Let's try
using this. Maybe it wasn't around at the time, this code is
more-or-less unchanged from I7d83bb2b359e7a8c3858eca04c96e35cf4e1fe9e
Change-Id: Ibaaa81124098f3c6febe48e455d3e1cd0a5f1761
6c8d7432a6