Minimize delta to Debian's upstream/master

Sync as much as possible with what Debian uses, to
make merging against upstream easy and be as close
to Debian's behavior as possible.

Changes synced from Debian upstream:

* xz compression is the default nowadays with gbp
* the FIXME file (debian/lib/live/boot/FIXME) no longer exists
* Recommends for eject + file aren't relevant within live-boot
* Don't re-implement initramfs-tools' log_failure_msg + log_warning_msg,
  use their common behavior instead
* Debian's ip=... handling supports setting nameserver via its 5th field
* Drop our ip=... handling with copying complete ip=args into STATICIP
  variable, this doesn't work as expected with the current code

Remaining changes where we diverge from Debian:

* custom bootid=... + ignore_bootid handling
* support nodhcphostname boot option
* support vlan=... boot option
* display Grml specific information on boot failures
* support /etc/grml_version
* custom network handling via Grml_Networking
* we don't support images directly on block devices
  in find_livefs(), since this might raise unexpected
  behavior in data rescue/forensic use cases