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/lilygo_techo/TechoBoard.h

@@ -14,19 +14,11 @@
 #define REAL_VBAT_MV_PER_LSB (VBAT_DIVIDER_COMP * VBAT_MV_PER_LSB)
 
 class TechoBoard : 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 "LilyGo T-Echo";
   }