mavlink: store instances in array

- simplify mavlink header writes that go back through the mavlink module
 - add locking around mavlink command line usage (start, stop, status, etc)

src/modules/mavlink/mavlink_log_handler.h

@@ -41,9 +41,10 @@
 #include <time.h>
 #include <stdio.h>
 #include <cstdbool>
-#include "mavlink_bridge_header.h"
 #include <drivers/drv_hrt.h>
 
+#include "mavlink_bridge_header.h"
+
 class Mavlink;
 
 // MAVLink LOG_* Message Handler