In order to run a program at startup, in your case a Python script. You will need to create a service to be loaded when the board boots. The image you are using (Ublinux) uses init.d and systemd, but I'm not sure which would be the best approach for your image. If you want, you can contact the guys from EmutexLabs for more information about this image.
If you want to use systemd, you need to create a service with the structure of [Unit] [Service], take a look at this thread where there are other makers that have been done this. (This was made for Yocto but it will help you too.)
If you want to use init.d, there is a good tutorial you could find helpful. marcwebbie/passpie · GitHub
If you create a service with one of these methods, you will not need to start a session or anything to be able to have the script running.