'flash erase_check' command runs a check algorithm on a target
if possible. The algorithm is run repeatedly for each flash sector.
Unfortunately every start and stop of the algorithm impose not negligible
overhead.
In practice it means checking is faster than plain read only for
sectors of size approx 4 kByte or bigger. And checking sectors
as short as 512 bytes runs approx 4 times slower than plain read.

The patch changes API call target_blank_check_memory() and related
to take an array of sectors (or arbitrary memory blocks).

Changes in target-specific checking routines are kept minimal.
They use only the first block from the array and process it by
the unchanged algorithm.

default_flash_blank_check() routine repeats target_blank_check_memory()
until all blocks are checked, so it works with both multi-block
and single-block based checkers.

Change-Id: I0e6c60f2d71364c9c07c09416b04de9268807f5e
Signed-off-by: Tomas Vanek <vanekt@fbl.cz>
Reviewed-on: http://openocd.zylin.com/4297


Tested-by: jenkins
Reviewed-by: Andreas Bolsch <hyphen0break@gmail.com>