rockchip: evb_rk3399: Tidy up the README

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@chromium.org>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>
Reviewed-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>