Fix nr_srssim linking problems when building with E2 support

In this case, there are a number of linker errors:

    /usr/bin/ld: libL2_NR.a(rrc_gNB.c.o): in function `nr_rrc_transfer_protected_rrc_message':
    /home/richie/oai/openair2/RRC/NR/rrc_gNB.c:330:(.text+0x6f6): undefined reference to `signal_rrc_msg'

However, nr_srssim does not need RRC, while it is being linked in. Thus,
link without nr_rrc and L2. For the remaining two linker problems:

- provide a definition of NR_IF_Module_init() as in most other
  simulators
- move rrc_get_max_nr_csrs() to nr_common.c. It is already declared in
  the corresponding header, and itself does not have anything to do with
  RRC radio configuration, therefore it does not belong to
  nr_radio_config.c

common/utils/nr/nr_common.c

@@ -1447,6 +1447,31 @@ unsigned short get_N_b_srs(int c_srs, int b_srs) {
   return srs_bandwidth_config[c_srs][b_srs][1];
 }
 
+// TODO: Implement to b_SRS = 1 and b_SRS = 2
+long rrc_get_max_nr_csrs(const int max_rbs, const long b_SRS)
+{
+  if(b_SRS>0) {
+    LOG_E(NR_RRC,"rrc_get_max_nr_csrs(): Not implemented yet for b_SRS>0\n");
+    return 0; // This c_srs is always valid
+  }
+
+  const uint16_t m_SRS[64] = { 4, 8, 12, 16, 16, 20, 24, 24, 28, 32, 36, 40, 48, 48, 52, 56, 60, 64, 72, 72, 76, 80, 88,
+                               96, 96, 104, 112, 120, 120, 120, 128, 128, 128, 132, 136, 144, 144, 144, 144, 152, 160,
+                               160, 160, 168, 176, 184, 192, 192, 192, 192, 208, 216, 224, 240, 240, 240, 240, 256, 256,
+                               256, 264, 272, 272, 272 };
+
+  long c_srs = 0;
+  uint16_t m = 4;
+  for(int c = 1; c<64; c++) {
+    if(m_SRS[c]>m && m_SRS[c]<max_rbs) {
+      c_srs = c;
+      m = m_SRS[c];
+    }
+  }
+
+  return c_srs;
+}
+
 frequency_range_t get_freq_range_from_freq(uint64_t freq)
 {
   // 3GPP TS 38.101-1 Version 19.0.0 Table 5.1-1: Definition of frequency ranges