Looks like removing printf drops about 10kB.

It works! Unceremoniously bailing out on the WiFi driver without
shutting it down might not be great - handling the "GO" and "REBOOT"
commands gracefully in the main loop would probably be better.

But hey, it works!

...but not it's obvious - we can't assume each packet nicely
encapsulates a single stage, it might all arrive in one go.
So the current "recv" implementation can't work. d'oh!

Now isolated for easy re-use. Should split it into a separate library.

Based on the Pi example