NR UE scan for SSB within current bandwidth

Till this commit, the UE gets the position of SSB in frequency from
command line and does the syncronization by locating the SSB in time. It
does this by generating a PSS time signal during initialization with the
SSB offset given via command line. Then PSS detection is done by
correlation.

In this commit, the UE computes a list of GSCN in the current NR band
that falls within the current bandwidth for the set center frequency.
For each GSCN in the list, the UE starts a thread and performs the
following:
1. Generate PSS time signal with SSB offset for current GSCN.
2. Detect PSS, SSS and decode PBCH (this step remains the same).
After all threads finishes, the results are collected.

For now, only the first detected PBCH is used to proceed further. So
this PBCH is indicated to MAC and UE exits initial sync.

Future work: The cell search can be standalone feature where the UE
could scan for SSB in the entire NR band. This would require changing
radio's frequency on the fly as the UE's bandwidth would not cover an
entire NR band. The cell detection can futher be imporved by looking
for SIB1 for each decoded PBCH.

openair2/NR_UE_PHY_INTERFACE/NR_IF_Module.c

@@ -1038,6 +1038,7 @@ static int handle_bcch_bch(NR_UE_MAC_INST_t *mac,
   mac->mib_ssb = ssb_index;
   mac->physCellId = cell_id;
   mac->mib_additional_bits = additional_bits;
+  mac->ssb_start_subcarrier = ssb_start_subcarrier;
   if(ssb_length == 64)
     mac->frequency_range = FR2;
   else