Deduplicate reboot() for NRF52 boards

The reboot() method is the same for all NRF52 boards. Use a shared
implementation.

Signed-off-by: Frieder Schrempf <frieder@fris.de>

variants/rak4631/RAK4631Board.h

@@ -55,9 +55,5 @@ public:
     return "RAK 4631";
   }
 
-  void reboot() override {
-    NVIC_SystemReset();
-  }
-
   bool startOTAUpdate(const char* id, char reply[]) override;
 };