Bt discovery fix (#880)

* launchDiscovery fix for esp32 (sending the discovery mqtt messages on the main core)

main/config_BT.h

@@ -29,6 +29,8 @@
 extern void setupBT();
 extern bool BTtoMQTT();
 extern void MQTTtoBT(char* topicOri, JsonObject& RFdata);
+extern void emptyBTQueue();
+extern void launchBTDiscovery();
 
 #ifdef ESP32
 extern int btQueueBlocked;