spl: Add full fitImage support

Add support for loading U-Boot and optionally FDT from a fitImage
in SPL by using the full fitImage support from U-Boot. While we do
have limited SPL loading support in SPL with a small footprint, it
is missing a lot of important features, like checking signatures.
This support has all the fitImage features, while the footprint is
obviously larger.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Pantelis Antoniou <pantelis.antoniou@konsulko.com>
Cc: Simon Glass <sjg@chromium.org>
Reviewed-by: Simon Glass <sjg@chromium.org>