Useful to have while doing handover-related changes.

/cc @jhuge, @lu.xu, @tomo, @xavier_thompson, @Daetalus
/proposed-for-review-on !1528
/reviewed-by TrustMe

Appendix. Diff for rendered enb.cfg and gnb.cfg before and after this patch

```
$ git diff --no-index config/{old,out}
```

```diff
diff --git a/config/old/enb.cfg b/config/out/enb.cfg
index 1843e0f24..43301ee13 100644
--- a/config/old/enb.cfg
+++ b/config/out/enb.cfg
@@ -45,6 +45,16 @@
       root_sequence_index: 204,
       dl_earfcn: 36100,
       inactivity_timer: 10000,
+      // Handover
+      ncell_list: [
+        // Inter-ENB HO
+        {
+          n_id_cell: 35,
+          dl_earfcn: 700,
+          cell_id: 0x12345,
+          tac: 123,
+        }],
+

       // Carrier Aggregation
       scell_list: [
diff --git a/config/old/gnb.cfg b/config/out/gnb.cfg
index d76b45d3c..2127a2f6b 100644
--- a/config/old/gnb.cfg
+++ b/config/out/gnb.cfg
@@ -57,6 +57,25 @@
     ssb_pos_bitmap: "10000000",

     inactivity_timer: 10000,
+    // Handover
+    ncell_list: [
+      // Inter-ENB HO
+      {
+        rat: "nr",
+        dl_nr_arfcn: 520000,
+        ssb_nr_arfcn: ,
+        ul_nr_arfcn: 520000,
+        n_id_cell: 75,
+        gnb_id_bits: 22,
+        nr_cell_id: 0x77712,
+        tac: 321,
+        band: 38,
+        ssb_subcarrier_spacing: 30,
+        ssb_period: 20,
+        ssb_offset: 0,
+        ssb_duration: 1,
+      }],
+
```