Previously, the flag did two things at once: enable support for using generic partitions as root fs if there were only one/allow use of partition-table-less images as root fs. And secondly, insist that there was a rootfs, and fail if not. Let's split these two in two separate options so that they can be used independently of each other. There are cases where one wants to use one without the other (i.e. when inspecting things with systemd-dissect tool it should be OK to do so even if image has no root fs), and it's cleaner anyway.
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