The SSB nFAPI message is only sent every 8 frames, and automatically
repeated by the PHY layer in between. This commit sets the vrb_map
structure in those frames in between nFAPI so that the scheduler does
not schedule on such resources.

This reverts commit 1a5874d4.

-Also change the place of the check on whether x2ap is enabled to trigger the ITTI tick message from RRC to X2AP.

-Since the aim of itti RRC_SUBFRAME_PROCESS message (MAC->RRC) is to trigger an itti X2AP_SUBFRAME_PROCESS message (RRC->X2AP), avoid creating
RRC_SUBFRAME_PROCESS message if x2ap task is not enabled. Not sure if this is the best solution. We could also avoid only sending the second itti X2AP_SUBFRAME_PROCESS message.

-Since the aim of itti RRC_SUBFRAME_PROCESS message (MAC->RRC) is to trigger an itti X2AP_SUBFRAME_PROCESS message (RRC->X2AP), avoid creating
RRC_SUBFRAME_PROCESS message if x2ap task is not enabled. Not sure if this is the best solution. We could also avoid only sending the second itti X2AP_SUBFRAME_PROCESS message.

The name NR_UE_list_t is misleading: beyond being a simple "list", this
structure carries all/most UE-related MAC information. If at all, it is
a container for multiple lists. The next commit introduces a
NR_UE_list_t structure that organizes UEs in lists such that it is
easier to go through the UEs.

when in phy test, only schedule UE to be in slot 8

The name NR_UE_list_t is misleading: beyond being a simple "list", this
structure carries all/most UE-related MAC information. If at all, it is
a container for multiple lists. The next commit introduces a
NR_UE_list_t structure that organizes UEs in lists such that it is
easier to go through the UEs.

when in phy test, only schedule UE to be in slot 8

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)

RLC has timers. We need to keep track of time. We chose to not use
realtime but 'processing time'. That is we send a tick to RLC for every
subframe when it is processed by the softmodem. Before this commit we
used to increase time in the function mac_rlc_status_ind which is called
for every subframe in the eNB, but not in the gNB (at least not in the
current gNB). So we need another way to keep track of time.

The function gNB_dlsch_ulsch_scheduler is called for every slot. It seems
to be the perfect candidate to send a tick to RLC for every subframe. We
take care of subcarrier spacing to send every subframe (one subframe is
always 1ms), not every slot (slot duration depends on subcarrier spacing).

It may not be the best solution. To be revised if needed.

Note: the UE has not been fixed. Some modification is needed for the UE
too, most probably.

I had to remove the Msg2 transmission in phytest mode, which was present in branch 'benetel_integration'.
This caused following assertion:

Assertion (Foffset <= Ncb) failed!
In nr_rate_matching_ldpc() /home/sli/sudas-demonstrator/openair1/PHY/CODING/nr_rate_matching.c:411
Foffset 6492 > Ncb 924