fit: Introduce methods for applying overlays on fit-load

Introduce an overlay based method for constructing a base DT blob
to pass to the kernel.

It is based on a specific method now to get the FDT from a FIT image
named boot_get_fdt_fit().

Signed-off-by: Pantelis Antoniou <pantelis.antoniou@konsulko.com>
Acked-by: Simon Glass <sjg@chromium.org>