Comment 5 for bug 1302418

So this actually turned out ot be very simple. I made the following change, and tested it in the following ways:

1) Boot in efi mode, do a use whole disk install. It created an efi system partition and used grub-efi.
2) Boot in efi mode, manually partition, remove the efi system partition, and replace it with bios_grub. It uses grub-pc, and booted fine in bios mode.
2) Installed 13.10 in bios mode, with an mbr partition table. Boot trusty in efi mode and do an along side install. It correctly used grub-pc, and boots fine in bios mode.

The patch is just this simple:

=== modified file 'grub-installer'
--- grub-installer 2014-03-28 13:11:31 +0000
+++ grub-installer 2014-04-08 16:22:27 +0000
@@ -442,6 +442,10 @@
  else
   grub_package="grub-efi"
  fi
+ if ! [ -d /target/boot/efi ]; then
+ #fall back to grub-pc
+ grub_package="grub-pc"
+ fi
  ;;
     powerpc/*)
  grub_package="grub-ieee1275"

I did notice two odd things in this process. The first is that when you manually partition the disk and don't have an esp, you get warned that you need one, but this warning does not happen when you do the guided install. I thought Colin mentioned on IRC that this warning should happen no matter how you get there. The second is that the menu option for the firmware setup that only applies for efi installs is still generated on the grub menu, even though we used grub-pc.