The community knows 
You have to add another software site to the target platform
- open project infrastructure->launch
- double click openhab.target
- click on Locations->Add
- click add->software site
- select Software Site->Next
- click Add
Enter any name
site: http://eclipse.github.io/smarthome/third-party/target/repository
ok - open Maven osgi bundles
- scroll down the list
- checkmark all tec.unon.* and Units of Measurement*
Finish - Click Reload Target Platform in the upper right corner
Save - Rebuild starts
- Open project Runtime->openhab.core.karaf
- double click on the pom.xml
- click on the red message “Plugin execution not covered…”
click “Mark goal highest…”
Maybe some errors are remaining, but those are related to specific bundles and could be ignored / delete the associated projects.
and go