stm32f4discovery: use openocd for flashing

STM32F407 has 1 MB flash, you are only erasing first 256 kB.

http://www.st.com/st-web-ui/static/active/en/resource/technical/document/data_brief/DM00037955.pdf
http://www.st.com/web/catalog/tools/FM116/SC959/SS1532/PF252419