mkimage: Require a data file when auto-fit is used

When auto-fit is used, it is not valid to create a FIT without an image
file. Add a check for this to avoid a very confusing error message later
("Can't open (null): Bad address").

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Tom Rini <trini@konsulko.com>
1 file changed