IT'S FINALLY HERE...
THE OFFICIAL ROCKCHIP-LEGACY MULTIMEDIA INTEGRATION
After two years of using a separate script to enable the multimedia features in RK3399 Legacy Kernel, the whole framework has been incorporated to the official Armbian packaging system. The choice distro for this integration is Debian Buster (see FAQ at the end of this post about the reasons).
Download a Armbian Buster Legacy Desktop image for your board, and instal