Free BT memory if using only WiFi

Partially fixes https://github.com/arduino/nina-fw/issues/4.

Problem: default certificates are a lot (~70KB) and BT uses a lot of RAM even if just configured in menuconfig.
Avoid spurious certificates parsing and SSL errors by freeing bt memory if unused.

main/sketch.ino.cpp

@@ -132,6 +132,7 @@ void setupBluetooth() {
 }
 
 void setupWiFi() {
+  esp_bt_controller_mem_release(ESP_BT_MODE_BTDM);
   SPIS.begin();
 
   if (WiFi.status() == WL_NO_SHIELD) {