GCS_MAVLink: Don't forward MAVLink data on channels marked private, make private channels more private

libraries/GCS_MAVLink/GCS_Common.cpp

@@ -108,6 +108,10 @@ bool GCS_MAVLINK::init(uint8_t instance)
         return false;
     }
 
+    if (!serial_manager.should_forward_mavlink_telemetry(protocol, instance)) {
+        set_channel_private(chan);
+    }
+
     /*
       Now try to cope with SiK radios that may be stuck in bootloader
       mode because CTS was held while powering on. This tells the

libraries/GCS_MAVLink/MAVLink_routing.cpp

@@ -97,6 +97,11 @@ bool MAVLink_routing::check_and_forward(mavlink_channel_t in_channel, const mavl
         return true;
     }
 
+    // don't ever forward data from a private channel
+    if ((GCS_MAVLINK::is_private(in_channel))) {
+        return true;
+    }
+
     // learn new routes
     learn_route(in_channel, msg);