extmod/moduos_dupterm: Make mp_uos_dupterm_tx_strn() function reusable.

Function to actually spool output terminal data to dupterm object.

extmod/misc.h

@@ -27,6 +27,13 @@
 
 // This file contains cumulative declarations for extmod/ .
 
+#include <stddef.h>
 #include "py/runtime.h"
 
 MP_DECLARE_CONST_FUN_OBJ(mp_uos_dupterm_obj);
+
+#if MICROPY_PY_OS_DUPTERM
+void mp_uos_dupterm_tx_strn(const char *str, size_t len);
+#else
+#define mp_uos_dupterm_tx_strn(s, l)
+#endif