[bsc] Store the cell_identity in gsm_bts and set it on the SI3 and SI6

Transfering the cell_identity from BSC to MSC is required for the
on-waves.com support. Allow to set the cell_identity in the cfg
file and patch the system information tables to set it.
diff --git a/openbsc/src/openbsc.cfg.1-1 b/openbsc/src/openbsc.cfg.1-1
index acd6681..a8331dd 100644
--- a/openbsc/src/openbsc.cfg.1-1
+++ b/openbsc/src/openbsc.cfg.1-1
@@ -14,6 +14,7 @@
  bts 0
   type bs11
   band GSM900
+  cell_identity 1
   location_area_code 1
   training_sequence_code 7
   base_station_id_code 63