[mtouch/mmp] When copying a directory hierarchy, delete the target fi…

…le if it exists.

The copyfile method does not like copying files over an existing file, and
sometimes it may end up showing an error.

tools/common/FileCopier.cs

@@ -123,6 +123,12 @@ static CopyFileResult CopyFileCallback (CopyFileWhat what, CopyFileStep stage, I
 					else if (stage == CopyFileStep.Err) {
 						Log (1, "Could not copy the file '{0}' to '{1}'", source, target);
 						return CopyFileResult.Quit;
+					} else if (stage == CopyFileStep.Start) {
+						if (File.Exists (target) || Directory.Exists (target)) {
+							Log (1, "Deleted target {0}, it's not up-to-date", target);
+							// This callback won't be called for directories, but we can get here for symlinks to directories.
+							File.Delete (target);
+						}
 					}
 					return CopyFileResult.Continue;
 				} else {