Petr Stehlík

Then everybody using Ubuntu on PC is named Noah. You see? I'd think it's an incompatibility in Armbian of some sort but I am not skilled enough to debug systemd based initramfs issues.

Oh well, sudo modprobe overlayfs indeed adds the overlay filesystem to /proc/filesystems. I am so used to automagic loading of modules that I didn't think of this. My fault.
Thank you, goldfish_paris!
 
Though adding "overlayfs" to /etc/modules didn't help the overlayroot package to start working correctly. Something else is wrong.