Thanks for all the input !

After some days the USB - serial adapter arrived. But it had to stay in the envelope.

I tried to mount the written flash card and alter boot.scr
The truck booted up und continously tried to boot from network.
But i changed converted the boot.cfg right on the mounted SD card.
This may have been not the right war, because it didn't work.
After some attempts i did quit this.

Then i moved to read about extlinux.conf
I'm could not try this, there are too many questions:

It says, it's about booting from ext filesystem.
After putting firmware and partition archives to SD card, there's only one vfat partition.
Others reported they had two partitions with using extlinux method.
First guess was formatting the card to ext filesystem, but then i still didn't
have an idea about the second partition i was missing.

So i will wait and gather some information about this before i will have a next try.
It looks too promising not to try it.

Instead i opened the usb <-> uart adapter envelope and installed it this afternoon.

The information about using the right connectors came from here:
https://andre.blaatschaap.be/2013/01/cu … onnection/