Make osmo-pcu wait for BTS to become available at start-up time.

After the PCU socket becomes available, the BTS might send an
INFO_IND message with the 'ACTIVE' flag cleared. If this happens,
do not exit immediately, but keep retrying until an INFO_IND
message with the 'ACTIVE' flag arrives.

Note that this change only affects behaviour at process start-up time.
If the BTS switches from active to inactive state then osmo-pcu will
still exit. If this behaviour should be changed as well it could be
done in a follow-up patch.

Tested against osom-bsc + osmo-bts-virtual.

Change-Id: Ic42a5601a43b81d260721fef5d9fa52447f9d309
Related: OS#2689
diff --git a/src/bts.h b/src/bts.h
index 5679b98..f9f385b 100644
--- a/src/bts.h
+++ b/src/bts.h
@@ -173,6 +173,7 @@
  * we can start to compile pcu_vty.c with c++ and remove the split.
  */
 struct gprs_rlcmac_bts {
+	bool active;
 	uint8_t bsic;
 	uint8_t fc_interval;
 	uint16_t fc_bucket_time;