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/nano_g2_ultra/nano-g2.cpp

@@ -24,8 +24,7 @@ static void disconnect_callback(uint16_t conn_handle, uint8_t reason)
 
 void NanoG2Ultra::begin()
 {
-  // for future use, sub-classes SHOULD call this from their begin()
-  startup_reason = BD_STARTUP_NORMAL;
+  NRF52Board::begin();
 
   // set user button
   pinMode(PIN_BUTTON1, INPUT);

variants/nano_g2_ultra/nano-g2.h

@@ -36,16 +36,11 @@
 #define REAL_VBAT_MV_PER_LSB     (VBAT_DIVIDER_COMP * VBAT_MV_PER_LSB)
 
 class NanoG2Ultra : public NRF52Board {
-protected:
-  uint8_t startup_reason;
-
 public:
   void begin();
   uint16_t getBattMilliVolts() override;
   bool startOTAUpdate(const char *id, char reply[]) override;
 
-  uint8_t getStartupReason() const override { return startup_reason; }
-
   const char *getManufacturerName() const override { return "Nano G2 Ultra"; }
 
   void powerOff() override {