Added default temperature from ESP32 MCU and NRF52 MCU

Added NRF52Board.h and NRF52Board.cpp
Modified NRF52 variants to extend from NRF52Board to share common feature

src/helpers/NRF52Board.h

@@ -0,0 +1,12 @@
+#pragma once
+
+#include <Arduino.h>
+#include <MeshCore.h>
+
+#if defined(NRF52_PLATFORM)
+
+class NRF52Board : public mesh::MainBoard {
+public:
+  float getMCUTemperature() override;
+};
+#endif