No more creds.c

Set PICOWOTA_WIFI_SSID/PICOWOTA_WIFI_PASS environment variables or
CMake variables.

May regret this!

This will be required for including into apps, so break it out.

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!

Now whatever arrives will get stored, and then used up by as many stages
as possible.

...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!

Don't allow data which is too long.

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

Based on the Pi example