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

Tagging Balint for a review, to speak to / quantify the tradeoffs here between gzip and lzma (compressed size + time to read initramfs from bootloader, vs. time to decompress in kernel).

There's no support for this yet in livecd-rootfs + live-build, but this seems like an ideal use case for lz4 instead - compressed initrd created once server-side, and uncompressed many times
on each boot of the live image. But we're still a way out from being able to make such a change and shouldn't block on it for bionic.

Also, aligning with my previous feedback to @rcj, we should really not be shadowing the live-build default compression here, but instead ensure $INITRAMFS_COMPRESSION is unset for cases where live-build defaults (currently gzip; future, lz4?) DTRT.