We observed strange issues that (among other things) revealed that make clean
did not really make things as clean as it could. This is an attempt to clean up
various files generated.

This aligns the behavior of the build variables, providing a helper function to do the job.

This allows to choose, when running cmake, whether the bootloader will connect to an existing AP (default behavior)
or fire up its own (by passing -DPICOWOTA_WIFI_AP=1) by using the same provided credentials.
DHCP requests are handled, with these settings the subnet is 192.168.4.1/24.

No more creds.c

Set PICOWOTA_WIFI_SSID/PICOWOTA_WIFI_PASS environment variables or
CMake variables.

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!

...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