Good evening everyone,
First off thank you Matthias, Tomasz and Alexander for the quick responses, and thank you Tomasz especially for all the great work that you do on the Raspberry Pi platforms!
Alexander seems to have hit the nail right on the head:
The uImage can be created automatically by Genode when extending the 'build.conf' file in the build-directory with the following line:
RUN_OPT += --include image/uboot
After adding this line to my build.conf, the resulting uImage could be successfully loaded and executed on the mentioned Pi 3 B.
For reproducibility: I now loaded the Image with "fatload mmc 0:1 ${scriptaddr} uImage", and then started the execution with "bootm ${scriptaddr}", which resulted in the hello_tutorial being successfully run and everything being printed via UART.
Thanks again for the quick help and have a nice weekend!
Best Regards, Daniel Gunzinger.