Fix --grub and --syslinux handling

With commit 3664e4ae we removed an argument from the grub_option()
and syslinux_warning() callback functions, which was reported as
false positives by vulture tool.

Fixes regression from 3664e4ae, failing grml2usb execution with --grub
(and deprecated --syslinux) command line option with:

| TypeError: grub_option() takes 3 positional arguments but 4 were given

Thanks: Ralf Moll for the bugreport