AP_HAL: allow forcing of trigger_groups()

libraries/AP_HAL/RCOutput.h

@@ -376,6 +376,11 @@ public:
     */
     virtual void write_gpio(uint8_t chan, bool active) {};
 
+    /*
+      Force group trigger from all callers rather than just from the main thread
+    */
+    virtual void force_trigger_groups(bool onoff) {};
+
     /*
      * calculate the prescaler required to achieve the desire bitrate
      */