I now have built both the RT kernel and the current linuxcnc build and it works on the tinkerboard. The link below has my patches that I used and built. I included the .patch file for those who want to compile. I offered the DEB paches for those who want to just install. They are compiled for 4.19.34. armbian. As to bilding linuxcnc, you will need to clone the GIT of linuxcnc, then follow one of the guides for how tobuild it, DO NOT build the SIMULATOR if you plan to spin motors and i'm not sure about the difference bewteen normal and DEV versions.
linuxcnc.org/docs/devel/html/code/building-linuxcnc.html
one of many sites on how to build
github.com/LinuxCNC/linuxcnc
the git source that will work for tinkerboard using armbian and many others
forum.armbian.com/topic/10007-tinker-boa...-working-free-offer/
My patches, when I get around to it I'll offer an SD card image that has the FULL preempt RT kernel ready to go. when I get the GPIO working with linuxcnc I'll post a working image of everything. So all someone has to do is pop the SDCARD image into the tinkerboard and adjjust settings as needed and have a working cnc controller.
Attached file is the RT patch that works with armbian NEXT or 3.19 for those who want to roll there own.