Linux: added builtins to show devices and topics

list_devices will list virtual devices starting with "/dev/".
list_topics will list topics ("/obj/")

Signed-off-by: Mark Charlebois <charlebm@gmail.com>

src/drivers/device/vdev_posix.cpp

@@ -274,10 +274,20 @@ void px4_show_devices()
 	VDev::showDevices();
 }
 
+void px4_show_topics()
+{
+	VDev::showTopics();
+}
+
 const char * px4_get_device_names(unsigned int *handle)
 {
 	return VDev::devList(handle);
 }
 
+const char * px4_get_topic_names(unsigned int *handle)
+{
+	return VDev::topicList(handle);
+}
+
 }