Using "default" for bootdev will install grub on default disk.

The grub-installer/bootdev configuration value is used in a
file to automatically select the grub install device. It works
fine if the disk is always the same, e.g. /dev/sda, but does
not work well when using the same preseed file on systems with
different types of disks, e.g. /dev/vda and /dev/hda.

Setting grub-installer/bootdev to "default" will automatically
select the first disk for grub installation. It works as long
as there is only one choice of disk to install on and thus
avoids the "Please select /dev/... or none" question.

Signed-off-by: Cyril Brulebois <kibi@debian.org>