So finaly i had some time to investigate this, as you can see above, it was my first time compiling an Linux Image.
Thx to the Libreelec Wiki, its not so complicated.
I got VM Image of latest Ubuntu running on Virtual Box. Set all up:
cloned last Libreelec Master sources from git
cloned GitHub - npcomplete13/LibreELEC.tv: Just enough OS for KODI to another folder.
copied changed files from npcomplete 13 repo to the Master:
(click on "..changed files")
and finaly i found out witch uboot param it was (found in sources scripts/uboot_helper)
PROJECT=Rockchip DEVICE=RK3399 ARCH=aarch64 UBOOT_SYSTEM=skykirinx99 make Image
copied to sd card with Installation of LibreELEC [LibreELEC.wiki].
most things work now, samba is slow, so better use sftp, Harddrive is going offline if directly on usb3 port - so i use usb hub,
no vnc, no moonlight, Kodi Libreelec Addon library is not available. V9 videos works. Device not turning display off and doesn't start after shutdown.
Kodi HDMI CEC config is found in "Settings->System->Input->Peripheral Devices->CEC Adapter",
you can configure the behavior of turning on/off TV and others.
PS: first Compiling run took 7 hours...