thank you very much for your fast response.
Here the link in the forum with a basic discussion about how to compile the addon
The makemkv addon was already in times of openelec generated. In the makemkv forum there is a howto posted.
Escalade, 5Schatten and several others built it already for Krypton and older Leia Releases of Libreelec. Unfortunately this addons are not working with the newer releases anymore.
Makemkv for arm based devices is available.