we want to generate the GRUB menu without actually installing GRUB
(because Amazon supplies its own pv-grub), and each menu entry
requires "root (hd0)". For the first, allow boot.loader.grub.device
to be set to "nodev" to indicate that the GRUB menu should be
generated without installing GRUB. For the second, add an option
boot.loader.grub.extraPerEntryConfig to allow commands to be added
to each GRUB menu entry (in this case, "root (hd0)").
svn path=/nixos/trunk/; revision=22712