HDMI problem when replacing u-boot

[msx_23]

Hello, I already asked some questions in this thread:
https://www.olimex.com/forum/index.php?topic=2114.15

But because this might have become a bit confusing I just create a new thread with my question. With a standard computer monitor over HDMI I had no problems, only with smaller hdmi displays that have no sound (just DVI signal) I think the basic problem is as follows:

I use the official debian buster image "A20-OLinuXino-buster-base-20210318-122357.img" with a A20-LIME board. When booting the first time I get HDMI output on u-boot but not in Linux. When rebooting both work as expected and I also get HDMI output on Linux. This is not ideal because the first boot fails in that sense but I could live with it.

But now i want to get rid of all HDMI outputs in u-boot and just start with a custom GUI application on the monitor once linux is started. I could successfully build and replace u-boot (and SPL) with the following github repo with default settings:

https://github.com/OLIMEX/u-boot-olinuxino

But when I replace the default u-boot the HDMI output in linux doesn´t work any more (no signal at display). In u-boot it still works. So I assume linux requires some hdmi preparation that is handled by u-boot and this was changed with the custom u-boot build.

So my question is how can I get rid of the u-boot hdmi output while the linux output should still work?

What are the steps/settings that are used for building the u-boot in the default .img-file?

Any help would be appreciated