I should add that syslinux isn't limited to booting Knoppix, even if it comes up with Knoppix screen. I just copied kernel and initrd for Debian 8.3 into the syslinux directory, added entry for Debian 8.3, and placed the compressed and persistent images in right places. No problem multi-booting Debian 8.3 and Knoppix. For more routine multiboot use, I prefer legacy grub, though.