[U-Boot] rockchip: make_fit_atf: Use BL31 environ variable for file location

[Philipp Tomsich]

> Other make_fit script (like imx or sunxi) use the BL31 environment
> variable to indicate the location of the file.
> Also do that for rockchip so we don't need to copy the file in the source
> directory.
> 
> Signed-off-by: Emmanuel Vadot <manu at freebsd.org>
> Reviewed-by: Philipp Tomsich <philipp.tomsich at theobroma-systems.com>
> ---
>  arch/arm/mach-rockchip/make_fit_atf.py | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 

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