Plane: ensure 10% free space when initialising logging

ArduPlane/Log.cpp

@@ -539,13 +539,15 @@ void Plane::log_init(void)
     if (!DataFlash.CardInserted()) {
         gcs_send_text_P(MAV_SEVERITY_WARNING, PSTR("No dataflash card inserted"));
         g.log_bitmask.set(0);
-    } else if (DataFlash.NeedErase()) {
-        gcs_send_text_P(MAV_SEVERITY_WARNING, PSTR("ERASING LOGS"));
-        do_erase_logs();
+    } else if (DataFlash.NeedPrep()) {
+        gcs_send_text_P(MAV_SEVERITY_WARNING, PSTR("Preparing log system"));
+        DataFlash.Prep();
+        gcs_send_text_P(MAV_SEVERITY_WARNING, PSTR("Prepared log system"));
         for (uint8_t i=0; i<num_gcs; i++) {
             gcs[i].reset_cli_timeout();
         }
     }
+
     arming.set_logging_available(DataFlash.CardInserted());
 }