Bug#820117: Setting grub-installer/bootdev via debconf-set does not work with d-i syntax

[Felix Winterhalter <felix@audiofair.de>]

Package: grub-installer
Version: 1.117+deb8u1

During a preseeded installation via netboot we set 
"grub-installer/bootdev" via debconf set during the pacman/early-command 
stage.

Setting this like in the preseed example (see: 
https://www.debian.org/releases/jessie/example-preseed.txt) via

debconf-set grub-installer/bootdev string /dev/sda

fails. After debugging for a whole day and tinkering on the insides of 
grub-installer we found out that you need to drop the "string" part so:

debconf-set grub-installer/bootdev /dev/sda

works as expected. This is however either an error in the documentation 
or an error in the parser, in either case it is not the documented behavior.

We have not tried if setting it via the preseed file like:

d-i grub-installer/bootdev string /dev/sda

would actually work. We didn't however find any place where there should 
be a difference between the syntax in either case.