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/promicro/PromicroBoard.h

@@ -22,15 +22,12 @@
 
 class PromicroBoard : public NRF52Board {
 protected:
-  uint8_t startup_reason;
   uint8_t btn_prev_state;
   float adc_mult = ADC_MULTIPLIER;
 
 public:
   void begin();
 
-  uint8_t getStartupReason() const override { return startup_reason; }
-
   #define BATTERY_SAMPLES 8
 
   uint16_t getBattMilliVolts() override {