So I got my rpi rootfs working quite well, BUT|
I can't use the bpi because I'm not able to mount my btrfs based external storage disk.
I hope there will be a kernel update soon.
And then I ran into a real problem: my screen goes dark 5 minutes after the pi booted. No way reactivating it.
I can still log into the bpi via ssh, restart lightdm with no luck, see in the logs that lightdm is still active and accepting my login, but no signal via HDMI
any help appreciated. I noticed there is a rpi config.txt in boot, but I doubt fiddling around with it will change anything?