Use default_flash_blank_check, this will use the much faster
blank_check_memory handler if supported - 15x quicker on stm32f4.

Otherwise it will fall back to using the slower default_flash_mem_blank_check.

Change-Id: Ia231b3e95468c9e92594dbdbe1fa2d69e1506fc3
Signed-off-by: Spencer Oliver <spen@spen-soft.co.uk>
Reviewed-on: http://openocd.zylin.com/632
Tested-by: jenkins