config: evb-rk3399: enable make uboot.itb

Similar to firefly 3399 enable the ability to create a FIT image
with combined arm-trusted-firmware.

Signed-off-by: Peter Robinson <pbrobinson@gmail.com>
Acked-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Reviewed-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>