don't overwrite existing MANIFEST.MF if useDefaultManifestFile is set

The MANIFEST.MF might be generated by a previous step, e.g. the
bnd-maven-plugin and must not be overwritten if <useDefaultManifestFile>
is configured to true

Author: Erwin Tratar

org.sonatype.m2e.mavenarchiver/src/org/sonatype/m2e/mavenarchiver/internal/AbstractMavenArchiverConfigurator.java

@@ -430,6 +430,12 @@ private void reflectManifestGeneration(MavenProject mavenProject, MojoExecution
     Xpp3Dom originalConfig = mojoExecution.getConfiguration();
     Xpp3Dom customConfig = Xpp3DomUtils.mergeXpp3Dom(new Xpp3Dom("configuration"), originalConfig);
 
+    Xpp3Dom useDefaultManifestFile = customConfig.getChild("useDefaultManifestFile");
+    if (Boolean.parseBoolean(useDefaultManifestFile.getValue()) && manifestFile.exists()) {
+    	//<useDefaultManifestFile>true</useDefaultManifestFile> and Manifest exists -> don't overwrite
+    	return;
+    }
+    
     //Add custom manifest entries
     customizeManifest(customConfig, mavenProject);