I personally think this is opening a can of worms. Now it's just a list
of alternative root devices. But the kernel knows absolutely nothing
about these. When is it fine to try an alternative? Why did the first
one not work? Did we just not wait long enough? Or is it a failed RAID
device? Or is it an encrypted disk that needs setup? Or is it on NFS and
the network is not available (or we are lacking driver firmware)?
It could actually introduce security problems: if I know that a device
will fallback to an alternative root (under my control), I can try and
DOS the primary root.