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)

preparing l2_init_ue and l3_init_ue to make sure parameters are passed down. To be merged with Francesco's changes and tested.

- reworked configuration file and configuration for gNB (write directly to ASN1 structure from config module instead of passing intermediate structures)
- added prach procedures for gNB with configuration of PRACH L1/L2 parameters

But NR-RRC-Definitions.asn file only include the NR-RRC-Definitions DEFINITIONS section
If we include the NR-UE-Variables into asn file, it will can't be compile in asn tool
and also some of implict problem for new version asn file

-Amend do_ServingCellConfigCommon function