There are two methods that will work. The first is the link I posted above but that is limited to just enabling serial, ssh and wifi.
The best method is to simply deploy the RaspberryPi OS image, before using mender-convert, enabling ssh with raspi-config and setting up any other files you may need. Then use dd to take an image of the updated SDCard and use that as input to mender-convert. This is the standard mechanism of using a Golden Master image with Mender.
Some related links that may help clarify:
- Workflow of updates
- https://www.linuxjournal.com/content/linux-iot-development-adjusting-binary-os-yocto-project-workflow
Drew