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_mesh_solar/MeshSolarBoard.h

@@ -37,9 +37,5 @@ public:
     return "Heltec Mesh Solar";
   }
 
-  void reboot() override {
-    NVIC_SystemReset();
-  }
-
   bool startOTAUpdate(const char* id, char reply[]) override;
 };