Use mapdevfs to find the device of the root filesystem.

This no longer forces the device path for the root system to be a DASD.
Instead it infers the root device like on any other platform. (The
code is mostly the one lilo-installer uses.)

This requires sysconfig-hardware (>= 0.0.10+nmu3, or feature-equivalent)
in the target system to set the relevant DASDs online by using
configuration stored in the initrd. It is copied into the final system
by s390-sysconfig-writer.