GCS_MAVLink: added accept_packet() hook

this will allow vehicles to control whether packets are accepted by
GCS sysid or not

libraries/GCS_MAVLink/GCS.h

@@ -225,6 +225,10 @@ public:
 
 protected:
 
+    // overridable method to check for packet acceptance. Allows for
+    // enforcement of GCS sysid
+    virtual bool accept_packet(const mavlink_status_t &status, mavlink_message_t &msg) { return true; }
+    
     bool            waypoint_receiving; // currently receiving
     // the following two variables are only here because of Tracker
     uint16_t        waypoint_request_i; // request index

libraries/GCS_MAVLink/GCS_Common.cpp

@@ -985,7 +985,8 @@ void GCS_MAVLINK::packetReceived(const mavlink_status_t &status,
     if (msg_snoop != NULL) {
         msg_snoop(&msg);
     }
-    if (routing.check_and_forward(chan, &msg)) {
+    if (routing.check_and_forward(chan, &msg) &&
+        accept_packet(status, msg)) {
         handleMessage(&msg);
     }
 }