[U-Boot] [U-Boot, v2, 09/19] rockchip: evb_rk3399: Tidy up the README

[Philipp Tomsich]

> Add mention of a prerequisite needed to build the image. Also adjust the
> English wording in a few places.
> 
> Ideally this should move to using binman to produce images, and avoid the
> manual steps.
> 
> Signed-off-by: Simon Glass <sjg at chromium.org>
> Reviewed-by: Kever Yang <kever.yang at rock-chips.com>
> ---
> 
> Changes in v2: None
> 
>  board/rockchip/evb_rk3399/README | 33 ++++++++++++++++++++------------
>  1 file changed, 21 insertions(+), 12 deletions(-)
> 

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