mkimage: Use generic code for showing an 'image type' error

mkimage: Use generic code for showing an 'image type' error

The existing error code only displays image types which are claimed by a
particular U_BOOT_IMAGE_TYPE() driver. But this does not seem correct. The
mkimage tool should support all image types, so it makes sense to allow
creation of images of any type with the tool.

When an incorrect image type is provided, use generic code to display the
error.

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