sys/stdio: add stdio_none

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 (,$(filter stdio_rtt stdio_cdc_acm stdio_none,$(USEMODULE)))
     USEMODULE += stdio_uart
   endif
 endif

sys/stdio_none/Makefile

@@ -0,0 +1 @@
+include $(RIOTBASE)/Makefile.base

sys/stdio_none/stdio_none.c

@@ -0,0 +1,39 @@
+/*
+ * Copyright (C) 2019 Freie Universität Berlin
+ *
+ * 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       Dummy STDIO implementation that does nothing to preserve energy
+ *
+ * @author      Hauke Petersen <hauke.petersen@fu-berlin.de>
+ *
+ * @}
+ */
+
+#include "stdio_base.h"
+
+void stdio_init(void)
+{
+    /* do nothing */
+}
+
+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;
+    return (ssize_t)len;
+}