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/sensecap_solar/SenseCapSolarBoard.h

@@ -5,12 +5,8 @@
 #include <helpers/NRF52Board.h>
 
 class SenseCapSolarBoard : public NRF52Board {
-protected:
-  uint8_t startup_reason;
-
 public:
   void begin();
-  uint8_t getStartupReason() const override { return startup_reason; }
 
 #if defined(P_LORA_TX_LED)
   void onBeforeTransmit() override {