Add APT recipe for rk3399 SoC

• APT recipe for rk3399 SoC New variable sbc (single-board computer) is introduced for selecting the target device. This variable is needed for proper U-Boot files selection and should have the same name as U-Boot directory targeting to this device under /usr/lib/u-boot/:
• firefly-rk3399
• pinebook-pro-rk3399
• puma-rk3399
• rock-pi-4-rk3399
• rock64-rk3328
• rockpro64-rk3399

"rock-pi-4-rk3399" is used as a default target device.

Tested fixedfunction and HMI with RockPro64 board -- works like a charm.