Fix `generate_doc.sh` issue (#236)
Fix `generate_doc.sh` issue (#236)

Script genreate_docs.sh is looking for build directory existance;
but it should actually check `build/default` directory's contents.
After `make clean`, contents of dir `build/default` will be empty.

dronecore/DroneCore,dronecore/DroneCore,dronecore/DroneCore,dronecore/DroneCore