I suggest you look into our new version of mender-convert. It’s still in development but considerably easier to expand than the previous version.
To clarify, we have not integrated mender-convert (either version) with Ubuntu on Beaglebone so there is likely work to do to enable that. We would love to have that as a contribution if you are so inclined.
Also, it may be easier to create the golden image from just the partition as it is likely going to be significantly smaller. And just to confirm, this golden image was created from an Ubuntu image for t the Beaglebone Black, correct?
Drew