More efficient compression

Right now we use gz but xz has significantly lower file size AND much faster decompression.
Furthermore, Google has a new compression format called Brotli which could also be a good candidate.

Acceptance criteria

• Propose alternatives to gz and measure their performance tradeoffs in various configuration
  • Key criteria: File size of a rootfs of about 500 MB and decompression resource use (including time)
• Implement alternative as replacement for gz