Then I copy the right *.dtb file and boot.scr to the 1st disk of test.sdimg, and the RaspberryPi3B will boot OK
This is not the right way to fix it. Yes your device has booted but it is missing a Mender integration in the bootloader. So this would need to be resolved first before moving on trying things out in user-space.
I guess best resource we have on U-boot integrations are: