[ext] Add mpaland/printf submodule

.gitmodules

@@ -16,3 +16,6 @@
 [submodule "ext/gcc/libstdc++"]
 	path = ext/gcc/libstdc++
 	url = https://github.com/modm-io/avr-libstdcpp.git
+[submodule "ext/mpaland/printf"]
+	path = ext/mpaland/printf
+	url = https://github.com/modm-ext/printf.git

ext/mpaland/printf.h

@@ -0,0 +1,26 @@
+/*
+ * Copyright (c) 2019, Niklas Hauser
+ *
+ * This file is part of the modm project.
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/.
+ */
+// ----------------------------------------------------------------------------
+
+#pragma once
+
+// empty include header for printf.c
+#include <stdarg.h>
+#include <stddef.h>
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+void _putchar(char character);
+
+#ifdef __cplusplus
+}
+#endif