Checkout referenced submodules

[sensor-freak]

Currently for all submodules the branch master is checked out. Instead, the commits that are referenced by the super project should be determined and checked out.

Problem: This would work if the checkout action could read from private submodule repos. Currently this does not work, which seems to be a bug/feature of the checkout action. Once the repos have been made public, the checkoout may work as expected. In the meantime the submodules are checked-out using explicit steps in the activity file. These steps do not refer to the registered commits, but directly to the master branch of each submodule.

(Alternativly: is it possible to determine the SHA from thos commits and provide them in the script?)