This F1 message sends us the CellGroupConfig of the UE. Store it in case
we need it. For instance, at least in OAI, it is possible to make a
handover without a DRB (this might or might not be standard compliant).
However, in this case, we did never receive a UE context setup request,
which stores the CellGroupConfig. Hence, store the first CellGroupConfig
we receive, to be able to do e.g., handover.