fstab-generator: add new root=tmpfs option

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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