Rename NCS_VERSION variable to NSO_VERSION
We should in general try to refer to it as NSO and not NCS.
NSO_VERSION can be used as an input variable to the build-version make target. Be careful not to overwrite it. I don't think this should happen since we just override it in our local scope and don't export it back to the environment - I think. But try to be sure! Even if we do, it should really be the same value!?