in case CONFIG_MULTI_DTB_FIT is set and u-boot-dtb.imx
image is build, currently u-boot-dtb.bin is used for
generating the u-boot-dtb.imx binary, which is wrong, as
it contains only a dtb blob not the fit.blob
Use instead the u-boot-fit-dtb.bin for generating
u-boot-dtb.imx which contains the fit.blob.
Signed-off-by: Heiko Schocher <hs@denx.de>