sandbox: Add -j option to indicate a jump from a previous U-Boot

In order to support the 'go' command we allow the jumping U-Boot to pass its
filename to the new U-Boot image. This can then be used to delete that image
if required.

Reviewed-by: Simon Glass <sjg@chromium.org>
Tested-by: Che-Liang Chiou <clchiou@chromium.org>
Signed-off-by: Simon Glass <sjg@chromium.org>