Extend kickstart implementation and parser for %pack section

%pack is a new script section that gets run after the image
has been packed and moved to outdir. IMAGE_OUTDIR is exported
in the script's environment.

Also make %attachment section usable in all image types not only loop
[zypp] Simplify pattern unrolling in zypp backend
[loop] Implement btrfs support for loop images according https://fedoraproject.org/wiki/Anaconda/Kickstart#btrfs