stm8 : new target

New STM8 target based mostly on mips4k. Target communication
through STLINK/SWIM. No flash driver yet but it is still possible
to program flash through load_image command. The usual target debug
methods are implemented.

Change-Id: I7216f231d3ac7c70cae20f1cd8463c2ed864a329
Signed-off-by: Ake Rehnman <ake.rehnman@gmail.com>
Reviewed-on: http://openocd.zylin.com/3953
Tested-by: jenkins
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>
Reviewed-by: Paul Fertser <fercerpav@gmail.com>