The official kodi wiki suggests using lightdm with the autologin enabled in order to autostart kodi. But there is a problem with this approach: If kodi crashes (which it frequently does), you are greeted with the login screen instead of having kodi restart.

What you would need is a kiosk mode for lightdm - which is awefully heavy in configuration - at least in comparison with nodm:

• Simply apt install nodm
• then edit /etc/default/nodm and change NODM_USER=root for NODM_USER=kodi (or whatever kodi user you have)

Then, enable autostarting for the user:

$ mkdir -p ~/.config/autostart && ln -s /usr/bin/kodi ~/.config/autostart