Good news everyone:
I GOT IT!!! (somehow).
I followed the Instructions here:
with some modifications for my custom-board (adapting the C-File for u-boot and changing the Device-Tree for Linux.)
With this I was able to create my own SD-Card with Linux 3.19 and Debian 8 Jessie on it. And it works, well mostly. I have to get KDE working (No, I don't want to start a Desktop-War! I simply want KDE!). So I installed kdm with all following dependencies.
After that, I was able to boot to the login-screen with mouse, keyboard and even touch.
But when I enter a username and password, X seems to start, but crashes with several messages like:
vgaarb: this pci device is not a vga device
and after a while:
imx-ipuv3 2400000.ipu: Timeout waiting for DMFC FIFOs to clear
The login aborts and returns me to login-screen.
Is there some sort of graphics-driver I have to install first? Or some setting in the Kernel? Do you need further Information?