lv_math: lv_math_num_to_str: return with buf

lv_misc/lv_math.h

@@ -31,7 +31,13 @@ extern "C" {
 /**********************
  * GLOBAL PROTOTYPES
  **********************/
-void lv_math_num_to_str(int32_t num, char * buf);
+/**
+ * Convert a number to string
+ * @param num a number
+ * @param buf pointer to a `char` buffer. The result will be stored here (max 10 elements)
+ * @return same as `buf` (just for convenience)
+ */
+char * lv_math_num_to_str(int32_t num, char * buf);
 
 /**********************
  *      MACROS