As of right now there are no pre-built images, you have to build EmbeddedBOINC yourself. But no worries, it is not hard, all you need is some HDD space (let's say 15 gigs -ish), Docker and internet connection!
First you need to build cross tools. In the dockerbuilder directory just build the two docker images. docker build --pull -t uebo-cross-arm64 -f Dockerfile.arm64 . and the same for armhf. This will take some time, on Ryzen 2700 limited to 12 threads it runs about 20 minutes.
Now start your docker container via docker run --rm -ti -v pwd:/sources2 uebo-cross-arm64 and in the container cd into buildsteps directory. From there just issue ./002_buildpkgs.sh. Do the same with armhf, if you want both arm64 and armhf packages.
Copy sample.config into .config file and edit it to suit your needs! Variables BOINC* and UEBO* are passed as kernel arguments so if you are not doing a PXE boot or pre-made config file you can ignore those there.
Almost done! Now you just need to package all the stuff into initramfs. Issue ./003_mkimg.sh yourplatform. If you don't see your platform in the list, go into platform directory and make one! it is just a matter of selecting the right DTB, architecture (arm64 vs arm) and package list.
The final step is the easiest. The same way as any other linux distribution the EmbeddedBOINC needs to be booted. Just feed the generated initramfs*.zst and Image to U-Boot either via local SD card or via PXE and you're done! You can edit the kernel arguments as specified in README to have more default settings ready after you boot up.