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/heltec_t114/T114Board.h

@@ -44,10 +44,6 @@ public:
     return "Heltec T114";
   }
 
-  void reboot() override {
-    NVIC_SystemReset();
-  }
-
   void powerOff() override {
     #ifdef LED_PIN
     digitalWrite(LED_PIN, HIGH);