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/minewsemi_me25ls01/MinewsemiME25LS01Board.h

@@ -23,7 +23,6 @@
 
 class MinewsemiME25LS01Board : public NRF52Board {
 protected:
-  uint8_t startup_reason;
   uint8_t btn_prev_state;
 
 public:
@@ -42,8 +41,6 @@ public:
     return (ADC_MULTIPLIER * raw);
   }
 
-  uint8_t getStartupReason() const override { return startup_reason; }
-
   const char* getManufacturerName() const override {
     return "Minewsemi";
   }