trx_toolkit/clck_gen.py: turn CLCKGen's thread into a daemon

If the main thread crashes, the CLCKGen's thread would never stop.
It would also happen if the main thread terminates without calling
CLCKGen.stop().  Let's prevent this by creating a daemon thread.

Change-Id: I9d41c5baa25fa0a263758414a164c1bded25e04e

src/target/trx_toolkit/clck_gen.py

@@ -70,6 +70,7 @@ class CLCKGen:
 
 		# Initialize and start a new thread
 		self._thread = threading.Thread(target = self._worker)
+		self._thread.setDaemon(True)
 		self._thread.start()
 
 	def stop(self):
@@ -128,6 +129,9 @@ class Application(ApplicationBase):
 		self.clck = CLCKGen([self.link], ind_period = 51)
 		self.clck.start()
 
+		# Block unless we receive a signal
+		self.clck._thread.join()
+
 	def sig_handler(self, signum, frame):
 		log.info("Signal %d received" % signum)
 		if signum == signal.SIGINT: