ci/fastboot: Add a serial timeout to catch fastboot prompt failure.

The a530s will occasionally fail to make it to the fastboot prompt, with no other deltas between a working log and a log stalled waiting for that line to show up.

So, add a serial timeout (like the rpi boards do for similar reasons), and on timeout restart the run. We actually restart the whole serial watching process, because the SerialBuffer finishes itself on timeout. This should also help with the intermittent issue we've had where a power cycle causes the python serial module to throw an exception.

Tested with the gitlab-disabled db820c that never makes it to the fastboot prompt (I think it's one where we need a longer micro cable to connect it!) and saw successful boot looping to retry.