Replace ISOLINUX/SYSLINUX with GRUB on MBR and eltorito

To extra protect BIOS systems from physical attacks it makes sense to encrypt boot partition so that ISO kernel can't be easily altered and the security dongle doesn't necessarily have to carry kernel and initramfs, too. From what I know, SYSLINUX does not support opening LUKS devices, so GRUB MBR/Eltorito bootmodes are needed.

Aforementioned change is already included on my public fork.