Fixed broken DMF API collection handling

Signed-off-by: kaizimmerm <kai.zimmermann@bosch-si.com>

hawkbit-dmf-api/src/main/java/org/eclipse/hawkbit/dmf/json/model/ActionUpdateStatus.java

@@ -9,6 +9,7 @@
 package org.eclipse.hawkbit.dmf.json.model;
 
 import java.util.ArrayList;
+import java.util.Collection;
 import java.util.Collections;
 import java.util.List;
 
@@ -72,4 +73,12 @@ public class ActionUpdateStatus {
         return this.message.add(message);
     }
 
+    public boolean addMessage(final Collection<String> messages) {
+        if (this.message == null) {
+            this.message = new ArrayList<>();
+        }
+
+        return this.message.addAll(messages);
+    }
+
 }