fstab-generator: don't accept missing root=, but accept root=none

fstab-generator: don't accept missing root=, but accept root=none

And other non-device entries (like fstab does).

Mount whatever the user asked to be mounted on / on the kernel
command line. Do less sanity check and do *not* bail out
when the mount device looks strange or does not exist.

This basically makes the changes for deviceless filesystems
from yesterday unnecessary and is in line with what we do for
filesystems set up in fstab.

Remove some code that is now dead (reverting fb02a2775a65 and
b0438462).

[tomegun:
  - change patch title/description a bit.
  - don't touch the /usr logic, that would be a separate change and
    we don't currently have a convincing use-case for that.
  - don't bail out on /sys ro. This only makes sense in containers,
    where we would not be doing this anyway. If there is a use-case
    we could consider that as a separate patch.]