It probably isn’t the answer you are looking for, but there is official Ubuntu 18.04 support for the 64-bit Jetsons. If this is important, then you might consider going to 64-bit.
I have the same issue. I managed to update to 16.04 and the system boots fine on the JTK1 but I’m not getting any HDMI output.SSH works perfectly fine, however. I would like to use this old Jetson as a low power PC and it would be very convenient to get the ubuntu 18.04 to work on it. I have not yet tried to update through regular upgrade process since I’d first like to resolve the HDMI issues.
What I’d like to mention is, and if someone can help me better understand how things work with the specific Nvidia driver package and the sample root filesystem… I noticed there are armhf drivers available for download and they support a wide variety of graphics cards (as per the supported product list). I wonder if it would be possible to dist-upgrade to 18.04 and install the armhf 390 or whichever version of the drivers? Is there any possibility of that working?