[U-Boot] [U-Boot, v2, 1/2] rockchip: board: lion_rk3368: update README flash instructions

[Philipp Tomsich]

> Add a section to the README on how to flash the on-board eMMC
> with the rkdeveloptool.
> 
> Signed-off-by: Klaus Goger <klaus.goger at theobroma-systems.com>
> Signed-off-by: Philipp Tomsich <philipp.tomsich at theobroma-systems.com>
> ---
> 
> Changes in v2: None
> 
>  board/theobroma-systems/lion_rk3368/README | 24 ++++++++++++++++++++++--
>  1 file changed, 22 insertions(+), 2 deletions(-)
> 

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