Kconfig: Add u-boot.itb BUILD_TARGET for Rockchip

Add u-boot.itb BUILD_TARGET for Rockchip platform when SPL_LOAD_FIT
is being used.

This can get rid of building itb explicitly with 'make u-boot.itb'
so, from now all required images will build just by make.

Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>