image: Introduce fit_image_load() to load images from FITs

At present code to load an image from a FIT is duplicated in the three
places where it is needed (kernel, fdt, ramdisk).

The differences between these different code copies is fairly minor.
Create a new function in the fit code which can handle any of the
requirements of those cases.

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