nixos: iso-image: use syslinux bootloader for USB booting support

This changes the bootloader for iso generation from Grub to syslinux. In addition this adds USB booting support, so that "dd" can be used to burn the generated ISO to USB thumbdrives
instead of needing applications like UnetBootin.

Most of this is based on the ISO generation methods that is used for ArchLinux.
@lethalman these cannot be true by default, since bootImage, efiBootImage, isohybridMbrImage need to be filled as well for the assertions to pass. I still think it might be a good idea to just remove all of these options and maybe rename the file to `make-iso9660-syslinux-image.nix` to make clear this is not just any regular ISO.