Fix default variables

The vars NSO_IMAGE_PATH, PKG_PATH and NSO_VERSION are used in the Makefiles as make variables as well as environment variables in some places. To make them accessible to the shell, export the vars. I thought about exporting the vars only in the targets that actually require them, but that seems more error prone - likely to miss something and then it doesn't work as indented.

This MR also updates the other two skeletons.