param lib: Provide used index lookup

src/modules/systemlib/param/param.h

@@ -129,6 +129,14 @@ __EXPORT bool		param_used(param_t param);
  */
 __EXPORT param_t	param_for_index(unsigned index);
 
+/**
+ * Look up an used parameter by index.
+ *
+ * @param param		The parameter to obtain the index for.
+ * @return		The index of the parameter in use, or -1 if the parameter does not exist.
+ */
+__EXPORT param_t	param_for_used_index(unsigned index);
+
 /**
  * Look up the index of a parameter.
  *