[U-Boot] rockchip: make_fit_atf.py depends on u-boot

[Philipp Tomsich]

> 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 <yousaf.kaukab at suse.com>
> Signed-off-by: Andreas Färber <afaerber at suse.de>
> ---
>  Makefile | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 

Reviewed-by: Philipp Tomsich <philipp.tomsich at theobroma-systems.com>