Add 60s watchdog for nrf52

In order to restore after battery voltage sags too low

src/MeshCore.h

@@ -58,6 +58,7 @@ public:
   virtual uint8_t getStartupReason() const = 0;
   virtual bool getBootloaderVersion(char* version, size_t max_len) { return false; }
   virtual bool startOTAUpdate(const char* id, char reply[]) { return false; }   // not supported
+  virtual void loop() { /* no op */ }
 
   // Power management interface (boards with power management override these)
   virtual bool isExternalPowered() { return false; }