[bootloader] use '--force' argument

- Note (Kevin Kofler): Grub will fall back to a LILO-style installation,
  using blocklists to reference the blocks inside a larger partition,
  with the same caveats LILO had (need to reinstall whenever you do
  anything to the partition that will move those blocks around).
- Setting flags (EF00, EF02) or creating grub boot partition should be
  handled within 'partition' module.