Flat shading on Landscape without DDX DDY

Hi.
I want to make flat-shaded landscape for my Mobile game, like this one.

http://behance.vo.llnwd.net/profiles11/289221/projects/2581967/hd_95f50990fb7dc0ca6534ac08e44f8c61.jpg

I found a good example here: [Landscape-without-smoothing][2]. But DDX and DDY not supported on most of mobile devices. I also found fake flat shaded here: [Flat-shading-tips-techniques][3] but don’t know how to apply it on landscape.

I’m looking for cheap (maybe fake) flat shading method for mobile devices. This method may even have incorrect shading, all I need is sharp-hard edges between triangles. Then I can simply Lerp this flat triangles with real VertexNormal and get pretty good result.

I’ll like to know that too.