DataFlash: log the last message sample time for GPS

libraries/DataFlash/LogFile.cpp

@@ -743,7 +743,8 @@ void DataFlash_Class::Log_Write_GPS(const AP_GPS &gps, uint8_t i, uint64_t time_
         hacc          : (uint16_t)(hacc*100),
         vacc          : (uint16_t)(vacc*100),
         sacc          : (uint16_t)(sacc*100),
-        have_vv       : (uint8_t)gps.have_vertical_velocity(i)
+        have_vv       : (uint8_t)gps.have_vertical_velocity(i),
+        sample_ms     : gps.last_message_time_ms(i)
     };
     WriteBlock(&pkt2, sizeof(pkt2));
 }

libraries/DataFlash/LogStructure.h

@@ -66,6 +66,7 @@ struct PACKED log_GPA {
     uint16_t vacc;
     uint16_t sacc;
     uint8_t  have_vv;
+    uint32_t sample_ms;
 };
 
 struct PACKED log_Message {
@@ -720,9 +721,9 @@ Format characters in the format string for binary log messages
     { LOG_GPS2_MSG, sizeof(log_GPS), \
       "GPS2", "QBIHBcLLefefB", "TimeUS,Status,GMS,GWk,NSats,HDop,Lat,Lng,Alt,Spd,GCrs,VZ,U" }, \
     { LOG_GPA_MSG,  sizeof(log_GPA), \
-      "GPA",  "QCCCCB", "TimeUS,VDop,HAcc,VAcc,SAcc,VV" }, \
+      "GPA",  "QCCCCBI", "TimeUS,VDop,HAcc,VAcc,SAcc,VV,SMS" }, \
     { LOG_GPA2_MSG, sizeof(log_GPA), \
-      "GPA2", "QCCCCB", "TimeUS,VDop,HAcc,VAcc,SAcc,VV" }, \
+      "GPA2", "QCCCCBI", "TimeUS,VDop,HAcc,VAcc,SAcc,VV,SMS" }, \
     { LOG_IMU_MSG, sizeof(log_IMU), \
       "IMU",  "QffffffIIfBB",     "TimeUS,GyrX,GyrY,GyrZ,AccX,AccY,AccZ,ErrG,ErrA,Temp,GyHlt,AcHlt" }, \
     { LOG_MESSAGE_MSG, sizeof(log_Message), \