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/t1000-e/T1000eBoard.cpp

@@ -5,8 +5,7 @@
 #include <bluefruit.h>
 
 void T1000eBoard::begin() {
-  // for future use, sub-classes SHOULD call this from their begin()
-  startup_reason = BD_STARTUP_NORMAL;
+  NRF52Board::begin();
   btn_prev_state = HIGH;
 
   // Enable DC/DC converter for improved power efficiency

variants/t1000-e/T1000eBoard.h

@@ -6,7 +6,6 @@
 
 class T1000eBoard : public NRF52Board {
 protected:
-  uint8_t startup_reason;
   uint8_t btn_prev_state;
 
 public:
@@ -34,8 +33,6 @@ public:
   #endif
   }
 
-  uint8_t getStartupReason() const override { return startup_reason; }
-
   const char* getManufacturerName() const override {
     return "Seeed Tracker T1000-e";
   }