Hello @hkhauke,
could I ask You if Your Linux kernel is properly configured to possess a support for device-tree overlay?
I has not got experience with Linux kernel version 4.18, but previous versions behave in such way that there is not necessary folders without DeviceTree overlay support configured.
Moreover, DeviceTree overlay should be applied (according to its documentation) to fseudo-filesystem /config/ rather than /sys/kernel/config/device-tree/. Of course, this pseudo-filesystem has to be configured too.
Besh wishes,
Jan Konečný.