Follow these steps to configure Apt and install a kernel and Machinekit packages:

1) The kernel-naming convention used in these packages may change as experience is accumulated, especially with ARM-based systems. Be sure to check back here before installing a new kernel.
2) Do NOT try to install more than one machinekit-{kernel-flavour} package on the same partition. The packages now contain some kernel-flavour set files which used to be in the machinekit-dev package and they conflict with each other. If you require to switch between different kernels on the same partition, build a development RIP build and run from that.

8. Debian Stretch on armhf (BBB | RPi2/3 | Other armhf)