tools: mkimage: add new image type "fdt_legacy"

If the user select the image type "flat_dt" a FIT image will be build.
This breaks the legacy use case of putting a Flat Device Tree into a
legacy u-boot image.

Add a new image type "fdt_legacy" to build a legacy u-boot image
with a "flat_dt" type.

Link: https://lore.kernel.org/all/20221028155205.ojw6tcso2fofgnhm@pengutronix.de


Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>
Reviewed-by: Sean Anderson <seanga2@gmail.com>