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>

src/helpers/NRF52Board.h

@@ -8,5 +8,6 @@
 class NRF52Board : public mesh::MainBoard {
 public:
   float getMCUTemperature() override;
+  virtual void reboot() override { NVIC_SystemReset(); }
 };
 #endif