DataFlash: print FMT messages for wrapped logs

this ensures we have FMT messages in every log, even if the log was
wrapped and the FMT messages were overwritten

libraries/DataFlash/DataFlash_Block.h

@@ -95,6 +95,10 @@ private:
     uint16_t GetFilePage();
     uint16_t GetFileNumber();
 
+    void _print_log_formats(uint8_t num_types, 
+                            const struct LogStructure *structure,
+                            AP_HAL::BetterStream *port);
+    
 protected:
     uint8_t df_manufacturer;
     uint16_t df_device;

libraries/DataFlash/LogFile.cpp

@@ -396,6 +396,26 @@ void DataFlash_Class::_print_log_entry(uint8_t msg_type,
     port->println();
 }
 
+
+/*
+  print FMT specifiers for log dumps where we have wrapped in the
+  dataflash and so have no formats. This assumes the log being dumped
+  using the same log formats as the current formats, but it is better
+  than falling back to old defaults in the GCS
+ */
+void DataFlash_Block::_print_log_formats(uint8_t num_types, 
+                                         const struct LogStructure *structure,
+                                         AP_HAL::BetterStream *port)
+{
+    for (uint8_t i=0; i<num_types; i++) {
+        const struct LogStructure *s = &structure[i];
+        port->printf_P(PSTR("FMT, %u, %u, %S, %S, %S\n"),
+                       (unsigned)PGM_UINT8(&s->msg_type), 
+                       (unsigned)PGM_UINT8(&s->msg_len), 
+                       s->name, s->format, s->labels);
+    }
+}
+
 /*
   Read the log and print it on port
 */
@@ -408,6 +428,7 @@ void DataFlash_Block::LogReadProcess(uint16_t log_num,
 {
     uint8_t log_step = 0;
     uint16_t page = start_page;
+    bool first_entry = true;
 
     if (df_BufferIdx != 0) {
         FinishWrite();
@@ -438,6 +459,10 @@ void DataFlash_Block::LogReadProcess(uint16_t log_num,
 
 			case 2:
 				log_step = 0;
+                if (first_entry && data != LOG_FORMAT_MSG) {
+                    _print_log_formats(num_types, structure, port);
+                }
+                first_entry = false;
                 _print_log_entry(data, num_types, structure, print_mode, port);
                 break;
 		}