This is preparatory work to handle the UE context setup response from
the target DU. There are two cases:
- "Initial" UE context setup response (during connection setup): we need
to inform the CU-UP via an E1 message of the new bearers (GTP tunnel
update)
- F1 handover: we need to inform the CU-UP via an E1 message that there
are DRBs to reestablish
There are two different functions, e1_send_bearer_updates() and
cuup_notify_reestablishment(), to do this. Harmonize the code, before
implementing the actual handover logic in the next commit.