This enable the user or board config to override the parameters
passed to stm32_stlink.cfg.

Required to fix a incorrect working area bug with the stm32vldiscovery.

Change-Id: I40a4f7913ff37d577d44b1f23befccf0317080a1
Signed-off-by: Spencer Oliver <spen@spen-soft.co.uk>
Reviewed-on: http://openocd.zylin.com/597
Tested-by: jenkins
Reviewed-by: Freddie Chopin <freddie.chopin@gmail.com>

Change-Id: I71253c2090162b1214bbbb37396735bb9128f920
Signed-off-by: Spencer Oliver <spen@spen-soft.co.uk>
Reviewed-on: http://openocd.zylin.com/334
Tested-by: jenkins
Reviewed-by: Freddie Chopin <freddie.chopin@gmail.com>