uboot-phytec

rockchip: firefly-rk3399: add FIT for rk3399

Enable SPL_FIT_GENERATOR with path for it.
With this patch you can get u-boot.itb for rk3399-firefly with:
> make u-boot.itb

Signed-off-by: Kever Yang <kever.yang@rock-chips.com>
Reviewed-by: Mark Kettenis <kettenis@openbsd.org>
Tested-by: Mark Kettenis <kettenis@openbsd.org>
Acked-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Reviewed-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>