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

variants/rak4631/RAK4631Board.h

@@ -2,6 +2,7 @@
 
 #include <MeshCore.h>
 #include <Arduino.h>
+#include <helpers/NRF52Board.h>
 
 // LoRa radio module pins for RAK4631
 #define  P_LORA_DIO_1   47
@@ -28,7 +29,7 @@
 #define  PIN_VBAT_READ    5
 #define  ADC_MULTIPLIER   (3 * 1.73 * 1.187 * 1000)
 
-class RAK4631Board : public mesh::MainBoard {
+class RAK4631Board : public NRF52Board {
 protected:
   uint8_t startup_reason;