DataFlash: use AP_Mission singleton

libraries/DataFlash/DFMessageWriter.cpp

@@ -134,12 +134,6 @@ void DFMessageWriter_WriteSysInfo::reset()
     stage = ws_blockwriter_stage_init;
 }
 
-void DFMessageWriter_DFLogStart::set_mission(const AP_Mission *mission)
-{
-    _writeentiremission.set_mission(mission);
-}
-
-
 void DFMessageWriter_WriteSysInfo::process() {
     const AP_FWVersion &fwver = AP::fwversion();
 
@@ -188,6 +182,12 @@ void DFMessageWriter_WriteSysInfo::process() {
 }
 
 void DFMessageWriter_WriteEntireMission::process() {
+    const AP_Mission *_mission = AP::mission();
+    if (_mission == nullptr) {
+        _finished = true;
+        return;
+    }
+
     switch(stage) {
 
     case em_blockwriter_stage_init:
@@ -234,9 +234,3 @@ void DFMessageWriter_WriteEntireMission::reset()
     stage = em_blockwriter_stage_init;
     _mission_number_to_send = 0;
 }
-
-
-void DFMessageWriter_WriteEntireMission::set_mission(const AP_Mission *mission)
-{
-    _mission = mission;
-}