cfg: add STM32F3-DISCOVERY board support

Change-Id: I4a02e0504fc04ffc1238d9bb77ec05c1f781e7e8
Signed-off-by: Spencer Oliver <spen@spen-soft.co.uk>
Reviewed-on: http://openocd.zylin.com/876
Tested-by: jenkins

tcl/board/stm32f3discovery.cfg

+# This is an STM32F3 discovery board with a single STM32F303VCT6 chip.
+# http://www.st.com/internet/evalboard/product/254044.jsp
+
+source [find interface/stlink-v2.cfg]
+
+source [find target/stm32f3x_stlink.cfg]
+
+# use hardware reset, connect under reset
+reset_config srst_only srst_nogate