@tqk
I don't know. See page 10 of this thread for seaniko7's own account of his intentions.
Some of the earlier instructions relating to installing the preview images directed one to this repository which isn't empty, for the uImage:
https://github.com/intromatyk/public_kernel/tree/iMX508
I could never find it there, but, like yourself, I've only arrived on the scene comparatively late. It turns up in the latest image in the downloadable tar archive which has the boot loader. Since the instructions were progressively revised during the development of the images, these links aren't very visible any more.
I only installed Obuntu a couple of weeks ago. The only real problem I've had, which is admittedly a significant one given the dependence of linux package managers on an internet connection, is the loading of the wireless firmware which fails during the boot.
I'm still trying to sort this out, if anyone has found a solution to this particular problem.