There are no plans for a full OH 1.9 distro – add-ons only after 1.8. So if you checkout the 1.8 branch, you can do a full build like before and test that. Add-ons remain on the master branch, which get packaged with OH2 distros, assembled from multiple repos.
Yes, I think I understand it now.
First I compile the 1.8er Distro to get the core.
Then I compile the 1.9.X Master. Now I fish out yet my bindings from the .m2 directory.
Or go to bundles/binding/org.openhab.binding.<yourbinding>/target and find them there.
Ok, now I have a current openhab installation.
Runtime is 1.81 and the Addons are 1.9. Perfect.
Thank you watou.
You’re welcome, but please be aware that 1.8.1 will ship with add-ons on the 1.8 branch. What you’ve created is the “bleeding edge” that will not be the same as the 1.8.1 maintenance release. (You probably already know this; just saying it for the record.) 
Also, if you see any problems running add-ons from master under the 1.8.* runtime, please make a noise!
Yes, when I notice problems, I will write about it.
For me it’s important that I now understand the relationships between versions a little better.