I got the 2.5.x version built. First, you don’t need the core from github (for which only the 3.0.0 branch is needed). But that wasn’t my problem. As explained here:
https://community.openhab.org/t/eclipse-setup-for-developing-2-5-x-addon
the imports change in 3.0.0. I had the imports right for 3.0.0, not for 2.5.x. I changed:
org.openhab.core
to
org.eclipse.smarthome.core
and also had to change the unit test imports and add a few more imports that seem to be needed in 2.5.x (I copied what I found in the 2.5.x version of the foobot binding).