flash: add stm32f2x flash lock/unlock cmds

Change-Id: I35344cc47fa4f0a49c034455c5abf479faa0344a
Signed-off-by: Spencer Oliver <spen@spen-soft.co.uk>
Reviewed-on: http://openocd.zylin.com/988
Tested-by: jenkins
Reviewed-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>