MAVLink: enable separate MAVLink helpers

this makes the core MAVLink functions into library calls, instead of
being inlined. The resulting code size doesn't change, but it means we
can safely call MAVLink functions from multiple places without causing
undue code bloat

libraries/GCS_MAVLink/GCS_MAVLink.cpp

@@ -10,3 +10,5 @@ BetterStream	*mavlink_comm_1_port;
 
 // this might need to move to the flight software
 mavlink_system_t mavlink_system = {7,1,0,0};
+
+#include "include/mavlink_helpers.h"

libraries/GCS_MAVLink/GCS_MAVLink.h

@@ -8,6 +8,8 @@
 
 #include <BetterStream.h>
 
+#define MAVLINK_SEPARATE_HELPERS
+
 #include "include/ardupilotmega/version.h"
 
 // this allows us to make mavlink_message_t much smaller