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 +1,24 @@
+/*
+ * 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_
+
+#ifdef __cplusplus
+extern "C"
+{
+#endif
+
+typedef long _WriteFunction(struct _reent *r, int fd, const char *ptr, int len );
+
+void set_write_stdout(_WriteFunction *f);
+_WriteFunction *get_write_stdout();
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif /* _STDOUT_REDIRECT_H_ */