Oops, forgot to write about it. At the moment I'm not 100% sure but I think what I tried was:

setenv mmckernfile bootimg.../zImage..
run trymmckernboot

But I just saw U-boot saying "Starting..." and it stopped there.
I suppose that the kernel actually booted (until trying to mount root) but without any console=tty0 command line the poor guy didn't print any messages on the screen. But obviously I cannot tell for sure.

I don't even know if I set the atagaddr or not. The phone is now in a drawer at home and I'm at work. I'll bring it to work tomorrow and try it here..