See merge request oai/openairinterface5g!804

Develop Integration Branch : 2020 week 15

The following Merge Requests have been included :

* MR 789 : fix sync source configuration for eNB and B200-mini
* MR 795 : dft/fft in shared lib
* MR 795 : NR gNB SCF indication
* MR 798 : Improvements on the "default" LTE-4G scheduler

Merge remote-tracking branch 'origin/NR_RRCConfiguration_sync_source' into develop_integration_2020_w15_bis

Instead of looping over UEs and all DL config Req/HI DCI0, loop first
over all DL config Req/HI DCI0, and for each, match UE.  Lowers CPU
traffic for high traffic situations.

adding API for USRP function issue_stream_cmd so that we can do timed reads in the nr-ru. This functionality is only enabled using a new flag use_single_antenna_port_for_tdd in openair0_cfg

this flaf will also put TX and RX on the same TRX antenna port

further usrp time now synchronized on pps, even when not using gps

This refactors the preprocessor. The new preprocessor implements the
same scheduling strategy (Modified Round-Robin: give resources according
to round-robin, then group) but should be fairer by changing every time
the RR start UE (i.e., 1st TTI UE 0 starts, 2nd TTI UE 1, etc) when the
UE number is not a divisor/multiple of the RBG size (and the last RBG is
often smaller). This also removes all unneeded code of the preprocessor
such as allocate(), positioning(), etc.

* do not check rlc_ind()
* Simplify retransmission part and remove nb_rb_available var
* Simplify new transmission code
* Simplify fill_DLSCH_dci(), avoid unnecessary copies
* fix: first check No. RBs, then check CCE alloc