Allow changing write function of stdout (#304)

* Allow changing write function of stdout.

Required for stdout redirection.
Works on blocks, not chars - does _not_ use sdk_os_putc !
Should work even when linking with SPIFFS.

core/include/stdout_redirect.h

@@ -0,0 +1,45 @@
+/*
+ * Part of esp-open-rtos
+ * Copyright (C) 2016 Oto Petrik <oto.petrik@gmail.com>
+ * BSD Licensed as described in the file LICENSE
+ */
+
+#ifndef _STDOUT_REDIRECT_H_
+#define _STDOUT_REDIRECT_H_
+
+#include <sys/reent.h>
+
+#ifdef __cplusplus
+extern "C"
+{
+#endif
+
+typedef long _WriteFunction(struct _reent *r, int fd, const char *ptr, int len );
+
+/** Set implementation of write syscall for stdout.
+ *
+ *  Use this function to redirect stdout for further processing (save to file, send over network).
+ *  It may be good idea to save result of `get_write_stdout()` beforehand and call
+ *  it at the end of the supplied function.
+ *
+ *  NOTE: use NULL to reset to default implementation.
+ *
+ *  @param[in] f  New code to handle stdout output
+ *
+ */
+void set_write_stdout(_WriteFunction *f);
+
+/** Get current implementation of write syscall for stdout.
+ *
+ *  Save returned value before calling `set_write_stdout`, it allows for chaining
+ *  multiple independent handlers.
+ *
+ *  @returns current stdout handler
+ */
+_WriteFunction *get_write_stdout();
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif /* _STDOUT_REDIRECT_H_ */