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.
Only @edolstra can say, I think there's no reason to have them set to false by default.