Machinekit

Machinekit

Stretch and Buster armhf realtime kernel

Xenomai is not supported in Stretch or Buster, you should use a rt-preempt kernel

There are many kernels in this repository http://repos.rcn-ee.com/debian/pool/main/l/linux-upstream/

You will need to check that it specifies rt and is targetted at your board type.