commander preflight checks: new structure using events interface

- Reporter class that keeps track of results (list of events + whether
  arming is possible + health).
  And only reports when one or more checks change.

Tools/px4events/jsonout.py

@@ -25,6 +25,8 @@ class JsonOutput():
                 event_obj = {}
                 event_obj['name'] = e.name
                 event_obj['message'] = e.message
+                if e.type is not None:
+                    event_obj['type'] = e.type
                 if e.description is not None:
                     event_obj['description'] = e.description
                 args = []