seco-rk3399-c31: u-boot and linux meta-data updates
Main changes:
- add the u-boot recipe of the machine that packages Rokchip loader.bin, trust.img and uboot.img
- add the machine configuration allowing to build a preferred kernel - 5.10 or 4.19
- some fixes in linux-firmware recipe related to commented lines and splitting of distinct packages.
NOTE: kernel-module-nxp89xx does need to be added in this meta layer or common one, to have wireless devices support.