What SuperTramp83 said.
Libreboot doesn't read grub from the disk. Grub is inside libreboot. So you need to modify your grub.cfg inside libreboot using the cbfstool. It's easiest to flash new modified roms from inside trisquel. I'm assuming you flashed the x200 yourself, so you don't need to worry to much about flashing a rom and then it doesn't boot as you can just reflash it externally again.
