Make diskimage-builder work in Docker.

When running inside a Docker container, we cannot rely on devices in
/dev/mapper to be automagically created by udev, because we probably
don't have a udev at all.  To work around this, run dmsetup mknodes
after every kpartx run.

Change-Id: If7e30579224ce54c5ed26d08974d8293c144719a
Merge "Make diskimage-builder work in Docker."