Ensure that grub-install doesn't receive emtpy command line argument

We run grub-install once without --force and if that fails
try rerunning with --force. On the first run we pass an
empty argument though that grub-install assumes as install
device and fails with:

  /usr/sbin/grub-install: error: More than one install device?.

By using "--" as option to signify the end of command line
options we avoid this failure in the run without passing the
--force option.