yes, i know your wiki. There is a lot of helpful information, thanks for that.
The informations for building an image from the scratch are a little bit “scattered”, imho, so i think it could be helpful to have all detailed informations about this topic in one single post.
I also found that the default build scripts in the official repository always create an image with the u-boot and a pre-loader programming the DRAM to 1333MHz, but there is also an unused pre-loader for the 1600MHz DRAM. I tried the 1600 version and it’s working fine, at least with my board (I don’t know if the kernel re-programs the DRAM speed later; in this case, the preloader version doesn’t matter).
One information is not in your wiki afaik: the problem with the u-boot building with gcc 5.5. If building with gcc 5.5, the pre-loader runs, but the board crashes when u-boot should be started. Do you have any clue why gcc5.5 cannot compile the bootloader correcly?
Ubuntu 18.04 is working as well. Did you try to use netplan for interface configuration? I think it should also work similar to the “old” interface version (the yaml config file has another format, of course). I’'l try that next.