AP_Relay: add AP::relay() to get relay singleton

libraries/AP_Relay/AP_Relay.cpp

@@ -162,3 +162,11 @@ void AP_Relay::toggle(uint8_t relay)
     }
 }
 
+namespace AP {
+
+AP_Relay *relay()
+{
+    return AP_Relay::get_singleton();
+}
+
+}

libraries/AP_Relay/AP_Relay.h

@@ -48,3 +48,7 @@ private:
     AP_Int8 _pin[AP_RELAY_NUM_RELAYS];
     AP_Int8 _default;
 };
+
+namespace AP {
+    AP_Relay *relay();
+};