Integration 2020 wk48

MR932 : Nr sa ngap rrc wk42 
- Initial implementation of NGAP and RRC extensions for SA
 
MR956 : Correct use of ssb_position bitmaps  

MR958 : 5G NR Downlink DMRS Type 2
- Implementation of 5G NR Downlink DMRS type 2 (generation on gNB side, channel estimation with linear interpolation in frequency domain on UE side), needed for MIMO
  
MR961 : 5G NR UE TTI cleanup  
- Remove current_thread_id array, nr_tti_rx, nr_tti_tx, subframe_rx and subframe_tx

MR965 : fix a segdev