mk: fix mavgen dependency

The rule to generate mavgen headers need to depend on the rule to check
the submodules, otherwise they can execute in parallel and the
(mavlink) submodule be updated while the headers are generated.

mk/mavgen.mk

@@ -5,7 +5,7 @@ MESSAGE_DEFINITIONS := $(SKETCHBOOK)/modules/mavlink/message_definitions/v1.0
 MAVLINK_HEADERS := $(BUILDROOT)/libraries/GCS_MAVLink/include/mavlink/v1.0/ardupilotmega/mavlink.h $(wildcard $(BUILDROOT)/libraries/GCS_MAVLink/include/mavlink/v1.0/,*.h) $(wildcard $(BUILDROOT)/libraries/GCS_MAVLink/include/mavlink/v1.0/ardupilotmega,*.h)
 MAVLINK_OUTPUT_DIR := $(BUILDROOT)/libraries/GCS_MAVLink/include/mavlink/v1.0
 
-$(MAVLINK_HEADERS): $(MESSAGE_DEFINITIONS)/ardupilotmega.xml $(MESSAGE_DEFINITIONS)/common.xml
+$(MAVLINK_HEADERS): CHECK_MODULES $(MESSAGE_DEFINITIONS)/ardupilotmega.xml $(MESSAGE_DEFINITIONS)/common.xml
 	echo "Generating MAVLink headers..."
 	#goto mavlink module directory and run the most recent generator script
 	echo "Generating C code using mavgen.py located at" $(SKETCHBOOK)/modules/mavlink/