If I direct refer to font-awesome.css in _layouts page. it will work
font-awesome.css
With version 5.1.0 I had to reference all.css instead of fontawesome.css i.e.,
all.css
fontawesome.css
bundles.Add(new StyleBundle("~/bundles/fontawesome").Include( "~/Content/all.css", new CssRewriteUrlTransform() ));