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/nano_g2_ultra/nano-g2.h

@@ -48,8 +48,6 @@ public:
 
   const char *getManufacturerName() const override { return "Nano G2 Ultra"; }
 
-  void reboot() override { NVIC_SystemReset(); }
-
   void powerOff() override {
     // put GPS chip to sleep
     digitalWrite(PIN_GPS_STANDBY, LOW);