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>

src/helpers/NRF52Board.h

@@ -6,7 +6,12 @@
 #if defined(NRF52_PLATFORM)
 
 class NRF52Board : public mesh::MainBoard {
+protected:
+  uint8_t startup_reason;
+
 public:
+  virtual void begin() { startup_reason = BD_STARTUP_NORMAL; }
+  virtual uint8_t getStartupReason() const override { return startup_reason; }
   float getMCUTemperature() override;
   virtual void reboot() override { NVIC_SystemReset(); }
 };