target: Add verify_image_checksum command

This avoids the secondary binary search if the checksum is different

Change-Id: I986ba7687cea76f30e37a6bca58aabde18198263
Signed-off-by: Evan Hunter <ehunter@broadcom.com>
Signed-off-by: Matthias Welwarsky <matthias.welwarsky@sysgo.com>
Reviewed-on: http://openocd.zylin.com/2869


Tested-by: jenkins
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>
Reviewed-by: Paul Fertser <fercerpav@gmail.com>