logging: publish a message for PX4_{WARN,ERR} & store them to the ulog file

- ulog file message rate limited to 50Hz
- queuing with size 2
- this replaces the mavlink log message in the ulog
  (but the mavlink warnings & errors still go to the ulog)

src/modules/uORB/uORBMain.cpp

@@ -36,6 +36,7 @@
 #include "uORBManager.hpp"
 #include "uORB.h"
 #include "uORBCommon.hpp"
+#include <px4_log.h>
 
 extern "C" { __EXPORT int uorb_main(int argc, char *argv[]); }
 
@@ -78,6 +79,8 @@ uorb_main(int argc, char *argv[])
 			return -errno;
 		}
 
+		px4_log_initialize();
+
 		return OK;
 	}