725ad3b062
It's useful to be able to combine a regular /usr/ file system with a tmpfs as root, for an OS that boots up in volatile mode on every single boot. Let's add explicit support for this via root=tmpfs. Note the relationship to the existing systemd.volatile= option: 1. The kernel command line "root=/dev/… systemd.volatile=yes" will mount the specified root fs, and then hide everything at the top by overmounting it with a tmpfs, except for the /usr subtree. 2. The kernel command line "root=tmpfs mount.usr=/dev/…" otoh will mount a toot fs at the top (just like the case above), but will then mount the top-level dir of the fs specified in mount.usr= directly below it. Or to say this differently: in the first case /usr/ from the physical storage fs is going to become /usr/ of the hierarchy ultimately booted, while in the second case / from the physical storage fs is going to become /usr of the hierarchy booted. Philosophically I figure systemd.volatile= is more an option for "one-off" boots, while root=tmpfs is something to have as default mode of operation for suitable images. This is currently hard to test reasonably, since Dracut refuses to accept root=tmpfs. This needs to be addressed separately though.
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