DataFlash: add PrepForArming method

libraries/DataFlash/DataFlash.cpp

@@ -321,6 +321,11 @@ bool DataFlash_Class::should_log(const uint32_t mask) const
         }                                         \
     } while (0)
 
+void DataFlash_Class::PrepForArming()
+{
+    FOR_EACH_BACKEND(PrepForArming());
+}
+
 void DataFlash_Class::setVehicle_Startup_Log_Writer(vehicle_startup_message_Log_Writer writer)
 {
     _vehicle_messages = writer;

libraries/DataFlash/DataFlash.h

@@ -101,6 +101,8 @@ public:
 
     void setVehicle_Startup_Log_Writer(vehicle_startup_message_Log_Writer writer);
 
+    void PrepForArming();
+
     void EnableWrites(bool enable) { _writes_enabled = enable; }
     bool WritesEnabled() const { return _writes_enabled; }
 

libraries/DataFlash/DataFlash_Backend.h

@@ -59,6 +59,8 @@ public:
 
     virtual uint32_t bufferspace_available() = 0;
 
+    virtual void PrepForArming() { }
+
     virtual uint16_t start_new_log(void) = 0;
     bool log_write_started;
 

libraries/DataFlash/DataFlash_File.cpp

@@ -835,6 +835,13 @@ void DataFlash_File::stop_logging(void)
     }
 }
 
+void DataFlash_File::PrepForArming()
+{
+    if (logging_started()) {
+        return;
+    }
+    start_new_log();
+}
 
 /*
   start writing to a new log file

libraries/DataFlash/DataFlash_File.h

@@ -73,6 +73,8 @@ public:
 
     void vehicle_was_disarmed() override;
 
+    virtual void PrepForArming() override;
+
 protected:
 
     bool WritesOK() const override;