Maybe kobo kernel lacks support of devtmpfs (needed for udev). In my experience is easier to add support to devtmpfs to kobo kernel than to get a mainline kernel to work with kobo hardware.
First, I would check if the kernel support devtmpfs or this option can be enabled via menuconfig. If devtmpfs is not supported you have two options:
- use mdev + tmpfs to populate /dev
- backport devtmpfs to kobo kernel
If you want to backport devtmpfs here is an example that WarheadsSE did for OXNAS kernel in order to be able to run udev ->>
https://github.com/WarheadsSE/OX820-2.6-linux