sys: stdio_null: add null driver

Makefile.dep

@@ -403,7 +403,7 @@ ifneq (,$(filter newlib,$(USEMODULE)))
   ifeq (,$(filter newlib_syscalls_%,$(USEMODULE)))
     USEMODULE += newlib_syscalls_default
   endif
-  ifeq (,$(filter stdio_rtt stdio_cdc_acm,$(USEMODULE)))
+  ifeq (,$(filter stdio_cdc_acm stdio_null stdio_rtt,$(USEMODULE)))
     USEMODULE += stdio_uart
   endif
 endif

sys/stdio_null/Makefile

@@ -0,0 +1,5 @@
+ifeq ($(DEVELHELP),1)
+  $(warning STDIO disabled via stdio_null, but DEVELHELP enabled)
+endif
+
+include $(RIOTBASE)/Makefile.base

sys/stdio_null/stdio_null.c

@@ -0,0 +1,53 @@
+/*
+ * Copyright (C) 2019 Bas Stottelaar <basstottelaar@gmail.com>
+ *
+ * This file is subject to the terms and conditions of the GNU Lesser
+ * General Public License v2.1. See the file LICENSE in the top level
+ * directory for more details.
+ */
+
+/**
+ * @ingroup     sys
+ * @{
+ *
+ * @file
+ * @brief       STDIO null driver
+ *
+ * This file provides a null driver for STDIO that does not depend on anything.
+ *
+ * @author      Bas Stottelaar <basstottelaar@gmail.com>
+ *
+ * @}
+ */
+
+#include "stdio_base.h"
+
+#if MODULE_VFS
+#include "vfs.h"
+#endif
+
+#define ENABLE_DEBUG 0
+#include "debug.h"
+
+void stdio_init(void)
+{
+#if MODULE_VFS
+    vfs_bind_stdio();
+#endif
+}
+
+ssize_t stdio_read(void* buffer, size_t count)
+{
+    (void) buffer;
+    (void) count;
+
+    return 0;
+}
+
+ssize_t stdio_write(const void* buffer, size_t len)
+{
+    (void) buffer;
+    (void) len;
+
+    return 0;
+}