macx-xcode: Use correct replacement key for bundle identifier

The 'BUNDLEIDENTIFIER' key ensures that we generate an rfc1034
compatible identifier.

Change-Id: Ic5cefb8ae888d768dd793813e7ee3c23c9a5582a
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>
Reviewed-by: Andy Shaw <andy.shaw@digia.com>