Make the 'offset' parameter optional, if omitted simply start at the
beginning of the flash bank.

Additionally, check if the argument is out of bounds of the flash bank.

Change-Id: Id1959eee5c395666c35f26342c3c50134dd564e5
Signed-off-by: Marc Schink <openocd-dev@marcschink.de>
Reviewed-on: http://openocd.zylin.com/3858


Tested-by: jenkins
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>