Compress the kernel with xz

In my tests this gains us about 10MB.
3 jobs for compress-kernel in 23 minutes and 42 seconds (queued for 3 seconds)
Status Name Job ID Coverage
  Lint
passed shellcheck #5304

00:00:15

passed shfmt #5305

00:00:11

 
  Build
passed build #5306

00:23:26