Code review comment for lp://staging/~mwhudson/livecd-rootfs/live-server-gzip-initramfs

Zstd initrd support is not there yet in the kernel thus it is not easily backportable to older LTS releases. While lz4 compresses worse the speedup compared to gz is substantial and not all tools have to be changed before 18.04 GA.

I took @vorlon's "does not seem urgent" [1] as a soft-block before 18.04 GA but I would be happier to just add lz4 support and we can provide slightly bigger but faster-booting images.

[1] https://lists.ubuntu.com/archives/ubuntu-devel/2018-March/040265.html