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/thinknode_m1/ThinkNodeM1Board.cpp

@@ -21,8 +21,7 @@ static void disconnect_callback(uint16_t conn_handle, uint8_t reason) {
 }
 
 void ThinkNodeM1Board::begin() {
-  // for future use, sub-classes SHOULD call this from their begin()
-  startup_reason = BD_STARTUP_NORMAL;
+  NRF52Board::begin();
 
   Wire.begin();