Use common NRF52 begin() and deduplicate() startup reason init

Use a common begin() method that can be called from derived classes
to contain the shared initialization code.

Signed-off-by: Frieder Schrempf <frieder@fris.de>

variants/rak4631/RAK4631Board.h

@@ -30,12 +30,8 @@
 #define  ADC_MULTIPLIER   (3 * 1.73 * 1.187 * 1000)
 
 class RAK4631Board : public NRF52Board {
-protected:
-  uint8_t startup_reason;
-
 public:
   void begin();
-  uint8_t getStartupReason() const override { return startup_reason; }
 
   #define BATTERY_SAMPLES 8