Transceiver: Log when sending of CLK indications begins

When the logging category TRXCLK is set to info osmo-trx prints a
logline that informs about the sending of clock indications. In practice
this those log lines are often used to identify that osmo-trx and
osmo-bts are running properly, so it would be helpful, even in
productive use, if there would be an information in the log that the
sending of clock indications has begun. However, the regular printing of
the clock indication log line would soon flood the log. So, lets have an
addional log line that logs only once when the transceiver starts and
quickly informs at loglevel NOTICE that clock indications are now sent.

Change-Id: I6aa88943b76c9a2bf7aed60d6a3d325c1f27820a
Related: OS#2577

Transceiver52M/Transceiver.cpp

@@ -1053,6 +1053,8 @@ bool Transceiver::driveReceiveRadio()
     return false;
 
   if (mForceClockInterface || mTransmitDeadlineClock > mLastClockUpdateTime + GSM::Time(216,0)) {
+    if (mForceClockInterface)
+      LOGC(DTRXCLK, NOTICE) << "Sending CLOCK indications";
     mForceClockInterface = false;
     return writeClockInterface();
   }