Questions regarding bbone environment


as mentioned previously I am preparing a simple yocto layer for
beaglebone. I did not find any hints in the github repos
(image-builder?) so far so maybe someone can help me here:

* Where does the default uEnv.txt come from - of where can I 'steal' it
* Currently I can set dt-overlays only with mounted rootfs. I think to
make kernel params working I need some initrd containing the dtbo
files - a pointer would be appreciated.