[follow-up from bug 979002]

plan:

Get rid of /etc/default/grub_installdevice for i386-pc target.

use-case:

To avoid complications when device names are not stable (see bug above). For
example when drivers change or VMs are moved around.

Also, yast will only propose configurations where grub resides in the disk with
the /boot partition - so the assumption below should ideally always be true.

how:

/usr/lib/bootloader/grub2/install replaces missing entries with defaults:

- device = disk where /boot/grub2/i386-pc/ lives
- there are other entries like 'activate' or 'generic_mbr'; they can be dropped
  as they are not used anyway by /usr/lib/bootloader/grub2/install

So, basically yast should only create grub_installdevice when grub is installed
to a non-default device.