Niibe successfuly wrote Gnuk image to STM32 part of STM8S Discovery Kit, using "stm32loarder".
As shown in the picture, serial module (bought at Akiduki) was connected to STM32 with two pins (TX and RX). Note that the signal level is 3.3V, not RS-232C level.
Besides, the pin of BOOT0 needs to be pull-up-ed.
The software is available at github.com. You can get it by typing:
$ git clone git://github.com/jsnyder/stm32loader.git $
Here is another information in Japanese: