Comment 69 for bug 1396379

Ubiquity invokes the grub-installer bash script to perform the actual boot loader install stuff - it reads the the grub-installer/bootdev debconf value to know where to install the bootloader.

There is some logic in there to find a default boot device which I suspect is how it's picking the first partition of the first drive in the machine, but I'm not very good at debugging shell scripts so I couldn't find a reason as to why it would be using that logic in the first place since grub-installer/bootdev is already set correctly.