@agherzan It looks like there are some more problems. I got rid of the error messages I mentioned, but it still took a few minutes before my service was started. My service should start after multi-user.target (After=multi-user.target).
I started from scratch with downloading thud branch of poky and meta-raspberrypi.
In my bblayers.conf file I have these layers: meta, meta-poky, meta-yocto-bsp and meta-raspberrypi
In local.conf I have added MACHINE ?= "raspberrypi3"