no main-thread while making a port to a new MCU

Dear sirs, I tried to do everything which is described in the RIOT-porting guide, but I stuck at a very early level. The base-system seems to work. Everytime when I stop the MCU with the emulation probe I am in the idle-thread and I can single-step further on. The misterious thing is that it seems, that the main thread is not involved in the working flow. Only one character is seen by a terminal software per restart. Have you any idea what's going wrong here?

Best regards Bernhard