common/stm32/serialno: utoa_upper: Update and document return value

src/platforms/common/stm32/serialno.c

@@ -23,7 +23,7 @@
 
 char serial_no[DFU_SERIAL_LENGTH];
 
-static const char *utoa_upper(uint32_t value, char *const str, int base) __attribute__((unused));
+static char *utoa_upper(uint32_t value, char *const str, int base) __attribute__((unused));
 
 void read_serial_number(void)
 {
@@ -73,9 +73,10 @@ void read_serial_number(void)
  * @param[in] value Machine integer for conversion
  * @param[out] str Pointer to caller-managed buffer write the string into
  * @param[in] base Representation base between 2 and 36
- * @returns NULL on error (unsupported base), or same as str
+ * @returns NULL on error (unsupported base), or pointer into str,
+ *          advanced past the written part (for chaining; points to inserted NUL-terminator)
  */
-static const char *utoa_upper(uint32_t value, char *const str, const int base)
+static char *utoa_upper(uint32_t value, char *const str, const int base)
 {
 	static const char digits[] = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ";
 	static char buf[33] = {0};
@@ -106,5 +107,5 @@ static const char *utoa_upper(uint32_t value, char *const str, const int base)
 	}
 	str[total + 1] = '\0';
 
-	return str;
+	return str + total + 1;
 }