Comment 1 for bug 1642999

One other complication; we currently calculate the rootfs offset based on the disk-offset sorted order of the gadget.yaml named partitions. It lives just after the farthest out (which previously equaled the last) partition.

Now, because the farthest out won't necessarily be the last named partition, we have to still ensure that the rootfs gets written after the farthest out partition, *not* the last named partition.