The kernel sources have been taken from there, thanks to @akemnade: https://github.com/akemnade/linux/tree/kobo/merged-5.16
Commit: 40a69e57c8491d6a90224672e7501ca2d0022550

This was a nightmare, took my entire evening and a part of the night, but I'm glad that it works!

(We had some trouble with the pull request, so I merged the changes in one commit here)
Closes #2

Error code 6 is already used by https://github.com/Kobo-InkBox/rootfs/blob/2c45339df177aae26fcdc321484806ad5ec793b8/etc/init.d/inkbox_gui#L100-L103