Some reasoning as in previous commit.

In a previous commit, the functionality of handling IPv6 (and DNS names)
as part of the IPv4 was introduced (in IPv4 field for backwards
compatibility). This commit now removes superfluous configuration
fields:
- IPv6:  handled by IPv4 field
- active: users can just remove an entry
- preference: not needed

- Remove any FlexRAN code
- Cleanup config files
- Remove LFDS7, libyaml dependencies

This reverts commit 29268d0a.

This reverts commit 96554bef

The configuration files named "gnb*conf" have been modified.
Other configuration files were left unmodified.

This commit contains work done by Eurecom and by Benetel.

This commit introduces several enhancements in X2AP and RRC to deal
with ENDC. The main goal is to deal with UE's on/off and with various
errors.

The following work is included:

    - DC prep and DC overall X2 timers
    - X2 messages for UE release and DC timers' timeout

Open problems:

    - what to do when the UE sends an scg Failure? This commit removes
      the UE. And this removal may not be correctly done
    - UE removal in the gNB's PHY layer (a next commit is coming,
      but it is hackish and will need a rework)