SPL: FIT: rework U-Boot image loading

Currently the SPL FIT loader always looks only for the first image in
the /images node a FIT tree, which it loads and later executes.

Generalize this by looking for a "firmware" property in the matched
configuration subnode, or, if that does not exist, for the first string
in the "loadables" property. Then using the string in that property,
load the image of that name from the /images node.
This still loads only one image at the moment, but refactors the code to
allow extending this in a following patch.
To simplify later re-usage, we also generalize the spl_fit_select_index()
function to not return the image location, but just the node offset.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Lokesh Vutla <lokeshvuta@ti.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Tested-by: Heiko Stuebner <heiko@sntech.de>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>
Tested-by: Kever Yang <kever.yang@rock-chips.com>
Tested-by: Jagan Teki <jagan@openedev.com>