Makefile: Extend make clean with submodule sync which will save some developers

Makefile

@@ -176,6 +176,7 @@ clean:
 	@rm -rf build_*/
 	@(cd NuttX && git clean -d -f -x)
 	@(cd src/modules/uavcan/libuavcan && git clean -d -f -x)
+	@(git submodule sync)
 
 # targets handled by cmake
 cmake_targets = test upload package package_source debug debug_tui debug_ddd debug_io debug_io_tui debug_io_ddd check_weak \