[KICKOFF] Add minimal code to build for Rockchip-based Seco's boards

Add Rockchip machine and BSP recipes following Clea OS new structure for Scarthgap.

BSP info:

kernel -> 5.10.110 coming from Kirkstone release of Clea OS
U-Boot -> 2017.09 coming from Kirkstone release of Clea OS

This minimal support allows to power on the supported board with a standard WKS image and a rootfs without any support to HW acceleration.

The low level configuration files for the varius SOCs come from the following sources:

• https://github.com/JeffyCN/meta-rockchip/tree/scarthgap
• (internal) https://git.seco.com/clea-os/layers/seco/meta-seco-rk/-/tree/kirkston

Include SOCs:

• PX30
• RK3399
• RK356x
• RK3588

Supported SECO machines:

• C31
• D23
• E09