Shallow clone submodules. (#5279)

There's no need to fetch the entire submodule history if we only need the latest revision. Speeds up the download a fair bit.

Makefile

@@ -536,9 +536,9 @@ endef
 	cmp $(ROOT_DIR)/Makefile $(ROOT_DIR)/Makefile >/dev/null 2>&1; if [ $$? -gt 0 ]; then printf "$(MSG_NO_CMP)"; exit 1; fi;
 	# Check if the submodules are dirty, and display a warning if they are
 ifndef SKIP_GIT
-	if [ ! -e lib/chibios ]; then git submodule sync lib/chibios && git submodule update --init lib/chibios; fi
-	if [ ! -e lib/chibios-contrib ]; then git submodule sync lib/chibios-contrib && git submodule update --init lib/chibios-contrib; fi
-	if [ ! -e lib/ugfx ]; then git submodule sync lib/ugfx && git submodule update --init lib/ugfx; fi
+	if [ ! -e lib/chibios ]; then git submodule sync lib/chibios && git submodule update --depth 1 --init lib/chibios; fi
+	if [ ! -e lib/chibios-contrib ]; then git submodule sync lib/chibios-contrib && git submodule update --depth 1 --init lib/chibios-contrib; fi
+	if [ ! -e lib/ugfx ]; then git submodule sync lib/ugfx && git submodule update --depth 1 --init lib/ugfx; fi
 	git submodule status --recursive 2>/dev/null | \
 	while IFS= read -r x; do \
 		case "$$x" in \