Hi,
I realized that the “solution” with adding "KERNEL_DEVICETREE = “bcm2710-rpi-3-b-plus.dtb” probably wasn’t ideal. I lost my overlays, and I guess it was not the correct way to solve this.
I tried setting up a new project following this guide: Raspberry Pi 3 Model B/B+
The image I built this way worked, and I didn’t get these problems. I tried removing all my layers from my original project, but still got the " No match between boot and root partitions" and the “fw_printenv” problems. I compared which git commit was currently checked out on the different layers and checked out the same versions on my originally project. That helped. I think the eroors disappeared after changing the git commit of poky and rebuilding the image.
Originally I cloned the poky project with: git clone git://git.yoctoproject.org/poky --branch yocto-3.1.5 --single-branch
Now, when it is working, poky is on 9ee329c18fbe0c42eaf3d43657ea30591f79143b