Compress initramfs with xz

In my tests this gains us about 10MB.
3 jobs for master in 29 minutes and 54 seconds (queued for 2 seconds)
Status Name Job ID Coverage
  Lint
passed shellcheck #5327

00:00:18

passed shfmt #5328

00:00:14

 
  Build
passed build:secure #5329
secure

00:29:36