GRUB: remove erroneous quotes around kernelopts

kernelopts are used by grml-rescueboot to communicate kernel arguments
from the host's grub to the grub within the grml ISO. E.g., the host
grub boot entry will contains something like:

    kernelopts=" arg1 arg2 blacklist=foo etc "
    export kernelopts

Currently, grub within the grml ISO will pass this entire string as one
single argument to the kernel. Remove the quotes such that the arguments
are interpreted as intended.

Fixes https://github.com/grml/grml-rescueboot/issues/5