Code review comment for ~smoser/cloud-initramfs-tools:rooturl-fsimage

If we're touching this, I'd like to see if we can retain a path to the original squashfs file we download. For curtin, in particular, we can specify a squash:/path/to/image.squashfs and curtin can use unsquashfs to unpack the squash to $target which runs in parallel; for some systems with fast storage/cpu this can be much faster than the rsync copy.

https://code.launchpad.net/~raharper/curtin/+git/curtin/+merge/383814