DataFlash: correct nanf log output

libraries/DataFlash/DataFlash.h

@@ -210,6 +210,9 @@ public:
     void handle_log_send(class GCS_MAVLINK &);
     bool in_log_download() const { return _in_log_download; }
 
+    float quiet_nanf() const { return nanf("0x4152"); } // "AR"
+    double quiet_nan() const { return nan("0x4152445550490a"); } // "ARDUPI"
+
 protected:
 
     const struct LogStructure *_structures;

libraries/DataFlash/LogFile.cpp

@@ -594,7 +594,7 @@ void DataFlash_Class::Log_Write_POS(AP_AHRS &ahrs)
         lng            : loc.lng,
         alt            : loc.alt*1.0e-2f,
         rel_home_alt   : -home,
-        rel_origin_alt : ahrs.get_relative_position_D_origin(origin) ? -origin : nanf("ARDUPILOT")
+        rel_origin_alt : ahrs.get_relative_position_D_origin(origin) ? -origin : quiet_nanf(),
     };
     WriteBlock(&pkt, sizeof(pkt));
 }

libraries/DataFlash/examples/DataFlash_AllTypes/DataFlash_AllTypes.cpp

@@ -175,6 +175,9 @@ void DataFlashTest_AllTypes::Log_Write_TypeMessages_Log_Write()
                         3432345232233432   // uint64_t
         );
 
+    // emit a message which contains NaNs:
+    dataflash.Log_Write("NANS", "f,d,bf,bd", "fdfd",  dataflash.quiet_nanf(), dataflash.quiet_nan(), NAN, NAN);
+
     flush_dataflash(dataflash);
 
     dataflash.StopLogging();