Combine config.cfg and wifi.cfg?

[jackjansen]

While not particularly elegant the advantage would be that there is only a single configfile that is required. This means we may be able to automatically upload this file to the spiffs/littlefs filesystem during initial boot.

Should investigate how the Arduino file-upload mechanism works again, maybe the combining isn't needed and we can upload multiple files just as easily. Also investigate whether platformio has a similar mechanism.