rockchip: make_fit_atf.py depends on u-boot

rockchip: make_fit_atf.py depends on u-boot

u-boot.itb depends on u-boot-nodtb.bin, which in turn depends on u-boot.
u-boot.its from Rockchip make_fit_atf.py (used by {evb,firefly}-rk3399)
wants to read u-boot but is lacking this dependency, so that u-boot.itb
cannot be built in one go. Detect its use and add the missing dependency.

Reported-by: Yousaf Kaukab <[email protected]>
Signed-off-by: Andreas Färber <[email protected]>
Reviewed-by: Philipp Tomsich <[email protected]>