AP_ExternalAHRS_VectorNav: Update VectorNav sim to respond to receive…

…d commands

Aside from the RRG,1 call, the simulation should reply with exactly what was received as a receipt confirmation to allow the driver to proceed past init

libraries/SITL/SIM_VectorNav.cpp

@@ -213,11 +213,13 @@ void VectorNav::update(void)
         send_packet2();
     }
 
-    // Strictly we should send this in responce to the request
-    // but sending it occasionally acheaves the same thing
-    if (now - last_type_us >= 1000000) {
-        last_type_us = now;
-        nmea_printf("$VNRRG,01,VN-300-SITL");
+    const ssize_t n = read_from_autopilot(&receive_buf[0], ARRAY_SIZE(receive_buf));
+    if (n > 0) {
+        if (strncmp(receive_buf, "$VNRRG,01", 9) == 0) {
+            nmea_printf("$VNRRG,01,VN-300-SITL");
+        } else {
+            nmea_printf("$%s", receive_buf);
+        }
     }
 
 }

libraries/SITL/SIM_VectorNav.h

@@ -45,6 +45,8 @@ class VectorNav : public SerialDevice {
     uint32_t last_pkt2_us;
     uint32_t last_type_us;
 
+    char receive_buf[50];
+
     void send_packet1();
     void send_packet2();
     void nmea_printf(const char *fmt, ...);