When deploying image, check that the correct prompt returns

From e-mail from Michael Hudson:

https://validation.linaro.org/lava-server/scheduler/job/44661/log_file#entry12

During the writing of the test rootfs, the board spontaneously reboots.
Then our retry logic kicks in and tries to transfer the rootfs again,
but of course /mnt/root is not mounted now and so it fills up /.

Not sure exactly what to do about this, it's a pretty surprising way for
things to screw up. One thing that comes to mind is the fact that the
master image boots with a different prompt, if the download-retry logic
was careful to wait for the correct prompt before retrying, this would
not have happened.