simplify WiFi boot procedure to prepare for on-demand stack load

currently ```esp_wifi_init``` have to be called in ```app_main``` or
WiFi will fail to boot. When possible to boot later, code will be moved
into ```_esp_wifi_start``` to be executed when necessary

cores/esp32/main.cpp

@@ -8,15 +8,11 @@ extern "C" void initArduino();
 extern void loop();
 extern void setup();
 
-void startWiFi() __attribute__((weak));
-void startWiFi() {}
-
 void loopTask(void *pvParameters)
 {
     bool setup_done = false;
     for(;;) {
         if(!setup_done) {
-            startWiFi();
             setup();
             setup_done = true;
         }