Turn instructions into a makefile command, allowing Windows GUI kids to create a make target in Eclipse just for this

Makefile

@@ -216,6 +216,10 @@ $(NUTTX_SRC):
 checksubmodules:
 	$(Q) (./Tools/check_submodules.sh)
 
+.PHONY: updatesubmodules
+updatesubmodules:
+	$(Q) (git submodule update)
+
 #
 # Testing targets
 #

Tools/check_submodules.sh

@@ -6,7 +6,7 @@ if [ -d NuttX/nuttx ];
 	if [ "$STATUSRETVAL" == "" ]; then
 		echo "Checked mavlink submodule, correct version found"
 	else
-		echo "mavlink sub repo not at correct version. Try 'git submodule update'"
+		echo "mavlink sub repo not at correct version. Try 'make updatesubmodules'"
 		echo "or follow instructions on http://pixhawk.org/dev/git/submodules"
 		exit 1
 	fi
@@ -22,7 +22,7 @@ if [ -d mavlink/include/mavlink/v1.0 ];
 	if [ "$STATUSRETVAL" == "" ]; then
 		echo "Checked NuttX submodule, correct version found"
 	else
-		echo "NuttX sub repo not at correct version. Try 'git submodule update'"
+		echo "NuttX sub repo not at correct version. Try 'make updatesubmodules'"
 		echo "or follow instructions on http://pixhawk.org/dev/git/submodules"
 		exit 1
 	fi