Exposing GPS/UTC time APIs

Provides hooks for the internal APIs handling GPS time base for device
synchronization.
In addition to that 'set_system_time_utc(...)' API is added which have no
roots in the stack. Its just a helper for the application developer to
acquire UTC time base given the difference between TAI and UTC time
bases.

features/lorawan/LoRaWANInterface.cpp

@@ -190,3 +190,40 @@ lorawan_status_t LoRaWANInterface::set_device_class(const device_class_t device_
     Lock lock(*this);
     return _lw_stack.set_device_class(device_class);
 }
+
+lorawan_time_t LoRaWANInterface::get_current_gps_time()
+{
+    Lock lock(*this);
+    return _lw_stack.get_current_gps_time();
+}
+
+void LoRaWANInterface::set_current_gps_time(lorawan_time_t gps_time)
+{
+    Lock lock(*this);
+    _lw_stack.set_current_gps_time(gps_time);
+}
+
+lorawan_status_t LoRaWANInterface::set_system_time_utc(unsigned int tai_utc_diff)
+{
+    // do not lock here
+
+    // Adjust epoch for 1970 to 1980 (time for Unix epoch to GPS epoch)
+    lorawan_time_t u_time = time(NULL) + UNIX_GPS_EPOCH_DIFF;
+    // Adjust for leap seconds since 1980. TAI is always ahead of GPS by 19 seconds
+    u_time += (tai_utc_diff - 19);
+    lorawan_time_t cur_gps_time = get_current_gps_time();
+
+    if (cur_gps_time == 0) {
+        // GPS time is not set. Application needs to request a clock sync.
+        return LORAWAN_STATUS_SERVICE_UNKNOWN;
+    }
+
+    u_time += cur_gps_time;
+
+    set_time(u_time);
+
+    time_t now = time(NULL);
+    tr_info("System Clock set - (UTC) = %s", ctime(&now));
+
+    return LORAWAN_STATUS_OK;
+}