flash: pre-check flash unlock for stm32f2x

flash: pre-check flash unlock for stm32f2x

add checking of the current flash lock status before performing the unlock sequence (which would fail in an unlocked state)

Change-Id: I693294c9cd2f59e69cb5bf3338120052fd680b1e
Signed-off-by: Bruno FLEURETTE <bruno.fleurette@gmail.com>
Signed-off-by: Øyvind Harboe <oyvind.harboe@zylin.com>
Reviewed-on: http://openocd.zylin.com/363
Reviewed-by: Øyvind Harboe <oyvindharboe@gmail.com>
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>
Tested-by: jenkins
Reviewed-by: Mathias Küster <kesmtp@freenet.de>