mkimage: Allow a FIT to include an image of any type

At present FIT images are set up by providing a device tree source file
which is a file with a .its extension. We want to support automatically
creating this file based on the image supplied to mkimage. This means that
even though the final file type is always IH_TYPE_FLATDT, the image inside
may be something else.

Signed-off-by: Simon Glass <sjg@chromium.org>