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/wio-tracker-l1/WioTrackerL1Board.h

@@ -6,12 +6,10 @@
 
 class WioTrackerL1Board : public NRF52Board {
 protected:
-  uint8_t startup_reason;
   uint8_t btn_prev_state;
 
 public:
   void begin();
-  uint8_t getStartupReason() const override { return startup_reason; }
 
 #if defined(P_LORA_TX_LED)
   void onBeforeTransmit() override {