So I successfully managed to install Kodi inside Retropie 3.0 beta 4.
However, I faced only one minor (first world) problem. Kodi doesn’t read my advancedsettings.xml. On another Rpi2 I have installed MySQL-server and set up a shared library. This is working on several devices; my .xml is correctly read from iPad to laptop and desktop.
I turned on debugging inside Kodi and found this error:
DEBUG: Loading: libnfs.so.1
ERROR: Unable to load libnfs.so.1, reason: libnfs.so.1: cannot open shared object file: No such file or directory
DEBUG: Dll libnfs.so.1 was not found in path
ERROR: NFS: Error loading libnfs (HandleDyLoad).
After installing libnfs1 everything works like a charm: sudo apt-get install libnfs1
Maybe the devs of Retropie can add this required dll to the Kodi Experimental package, so users can access NFS shares.