initial UEFI support, using isohybrid as default now

* restricted to amd64 only
* requires xorriso >=1.1.6-2 on build host
* requires recent syslinux version supporting --uefi
  (2:4.04+dfsg-9 known to be working) on build host
* requires grub-efi-amd64-bin in chroot (>=Debian/wheezy)

Thanks: Ulrich Dangel <mru@grml.org>
Ack-ed by: Christian Hofstaedtler <ch@grml.org>