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/mesh_pocket/MeshPocket.h

@@ -10,14 +10,8 @@
 #define  MV_LSB   (3000.0F / 4096.0F) // 12-bit ADC with 3.0V input range
 
 class HeltecMeshPocket : public NRF52Board {
-protected:
-  uint8_t startup_reason;
-
 public:
   void begin();
-  uint8_t getStartupReason() const override { return startup_reason; }
-
-
 
   uint16_t getBattMilliVolts() override {
     int adcvalue = 0;