Prepare for handling pkg state with files. So it requires having the
path defined before declaring targets. In addition, it cleans up the
old git-download target.
Currently, due to the way emb6 is compiled in our build system, it is
not possible to deactivate certain warnings. This fix provides a
solution for that, by building the submodules in the package build step
instead of on their own, similar to how JerryScript is build.