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/ikoka_nano_nrf/IkokaNanoNRFBoard.h

@@ -51,10 +51,6 @@ public:
     return MANUFACTURER_STRING;
   }
 
-  void reboot() override {
-    NVIC_SystemReset();
-  }
-
   bool startOTAUpdate(const char *id, char reply[]) override;
 };