flash/nor/core, target: don't ask for working mem if no target algo

flash/nor/core, target: don't ask for working mem if no target algo

The command 'flash erase_check' showed the message
'Running slow fallback erase check - add working memory'
even in the case the target didn't implement blank_check_memory.

Change return code of target_blank_check_memory() in this case
and sense it in default_flash_blank_check() and show a message
without a request for working memory.

Change-Id: I7cf9bf77742964b4f377c9ce48ca689e57d0882f
Signed-off-by: Tomas Vanek <vanekt@fbl.cz>
Reviewed-on: https://review.openocd.org/c/openocd/+/6765
Tested-by: jenkins
Reviewed-by: Tim Newsome <tim@sifive.com>