[U-Boot] [U-Boot, v2, 2/2] rockchip: board: puma_rk3399: update README flash instructions

[Philipp Tomsich]

> Puma supports other boot sources then SD-Card. Update README to include
> the required steps.
> 
>  * how to package a SPI-NOR SPL
>  * how to flash eMMC with 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:
> - add missing ./configure step while building rkdeveloptool
> 
>  board/theobroma-systems/puma_rk3399/README | 51 +++++++++++++++++++++++++-----
>  1 file changed, 43 insertions(+), 8 deletions(-)
> 

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