spl: fit: simplify logic for FDT loading for non-OS boots

spl: fit: simplify logic for FDT loading for non-OS boots

To better support bootin through an ATF or OPTEE, we need to
streamline some of the logic for when the FDT is appended to an image:
depending on the image type, we'd like to append the FDT not at all
(the case for the OS boot), to the 'firmware' image (if it is a
U-Boot) or to one of the loadables (if the 'firmware' is an ATF, an
OPTEE, or some other image-type and U-Boot is listed in the
loadabled).

To achieve this goal, we drop the os_boot flag and track the type of
image loaded.  If it is of type IH_OS_U_BOOT, we append the FDT.

Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Acked-by: York Sun <york.sun@nxp.com>
Reviewed-by: Simon Glass <sjg@chromium.org>