[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/bsc_init.c b/openbsc/src/bsc_init.c
index 9fff4fe..c626415 100644
--- a/openbsc/src/bsc_init.c
+++ b/openbsc/src/bsc_init.c
@@ -916,6 +916,10 @@
 	type_4->lai = lai;
 	type_6->lai = lai;
 
+	/* set the CI */
+	type_3->cell_identity = htons(bts->cell_identity);
+	type_6->cell_identity = htons(bts->cell_identity);
+
 	type_4->data[2] &= 0xf0;
 	type_4->data[2] |= arfcn_high;
 	type_4->data[3] = arfcn_low;