26.7.1. SquashFS
SquashFS creation with mksquashfs does not take fixed sizes, and I have successfully booted from it, but it is readonly, which is unacceptable.
But then we could mount ramfs on top of it with OverlayFS to make it writable, but my attempts failed exactly as mentioned at OverlayFS.
This is the exact unanswered question: https://unix.stackexchange.com/questions/343484/mounting-squashfs-image-with-read-write-overlay-for-rootfs