splash: add support for loading splash from a FIT image

Enable support for loading a splash image from within a FIT image.
The image is assumed to be generated with mkimage -E flag to hold
the data external to the FIT.

Signed-off-by: Tomas Melin <tomas.melin@vaisala.com>
Acked-by: Igor Grinberg <grinberg@compulab.co.il>