cdrom: Compress file system image using Zstandard

cdrom: Compress file system image using Zstandard

This patch uses the new Zstandard algorithm to compress the file system
image on the ISO image. This comes with these advantages:

* Compression is about twice as fast than XZ with the parameters we have
  selected here
* We use a lot less memory during compression and can therefore utilise
  all processor cores of the build machines
* Decompression (when installing IPFire and when creating the
  flash-image) is substantically faster

The downside is that the generated ISO image is slighty larger (~10MiB)
which I am okay with as a trade-off for the points mentioned above.

Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>
Signed-off-by: Arne Fitzenreiter <arne_f@ipfire.org>