Simplify Dracut cmdline script

We don't actually need a real value for the root kernel param, and
requiring one causes problems for things like the discovery ramdisk
that don't pass in a disk= parameter.  Dracut seems to be happy to
take /dev/zero as the value, so we can just always use that.

Change-Id: Ia724f0214c26aa18c6f8f41f2c48d7f25b52ee6c
Merge "Simplify Dracut cmdline script"