Comment 4 for bug 1491894

Ok, just trying to be a bit clearer for my own good :

A system upgraded from Lucid to Precise will have the following debconf setup :

* grub-pc/install_devices:
  grub-pc/install_devices_failed_upgrade: true
* grub-pc/install_devices_empty: true
  grub-pc/install_devices_failed: false
  grub-pc/install_devices_disks_changed:

Once this system will be upgraded to Trusty, it will have :

* grub-pc/install_devices_empty: true
  grub-pc/install_devices_failed_upgrade: true
  grub-pc/install_devices_disks_changed:
  grub-pc/install_devices_failed: false
* grub-pc/install_devices:

Normally, the system should have what a fresh precise system had in the previous comment :

* grub-pc/install_devices: /dev/vda
  grub-pc/install_devices_failed_upgrade: true
  grub-pc/install_devices_empty: false
  grub-pc/install_devices_failed: false
  grub-pc/install_devices_disks_changed: