rockchip: evb-rk3399: correct README for board bring up

%s/rkflashtool/rkdeveloptool/

We are using rkdeveloptool not rkflashtool.
Signed-off-by: NHeinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: NPhilipp Tomsich <philipp.tomsich@theobroma-systems.com>
Acked-by: NPhilipp Tomsich <philipp.tomsich@theobroma-systems.com>
Reviewed-by: NKever Yang <kever.yang@rock-chips.com>

board/rockchip/evb_rk3399/README

@@ -65,7 +65,7 @@ Compile the U-Boot
 Compile the rkdeveloptool
 =======================
   Follow instructions in latest README
-  > cd ../rkflashtool
+  > cd ../rkdeveloptool
   > autoreconf -i
   > ./configure
   > make