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@theobroma-systems.com> Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com> Reviewed-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com> Acked-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>