AP_HAL_Linux: allow PeriodicThread to stop

This takes the simplest approach of just waiting for the next time we
will process events.

libraries/AP_HAL_Linux/Thread.cpp

@@ -235,7 +235,7 @@ bool PeriodicThread::_run()
 
     uint64_t next_run_usec = AP_HAL::micros64() + _period_usec;
 
-    while (true) {
+    while (!_should_exit) {
         uint64_t dt = next_run_usec - AP_HAL::micros64();
         if (dt > _period_usec) {
             // we've lost sync - restart
@@ -248,6 +248,20 @@ bool PeriodicThread::_run()
         _task();
     }
 
+    _started = false;
+    _should_exit = false;
+
+    return true;
+}
+
+bool PeriodicThread::stop()
+{
+    if (!is_started()) {
+        return false;
+    }
+
+    _should_exit = true;
+
     return true;
 }
 

libraries/AP_HAL_Linux/Thread.h

@@ -80,6 +80,8 @@ public:
 
     bool set_rate(uint32_t rate_hz);
 
+    bool stop() override;
+
 protected:
     bool _run() override;