I tried googling and searching on stack but I didn\'t find anything :-(
( Mapping irregular shapes or other polygons (cartoons, sprites) to triangles in OpenGL ES )
I haven't tried using OpenGL ES, but judging from a quick look in the documentation you should be able to draw a convex polygon using e.g. a "triangle fan":
glVertexPointer(2, ..., arrayOfCoordinates)
...
glDrawElements(GL_TRIANGLE_FAN, ... , arrayOfIndices);
You can think of a "triangle fan" as the spokes of a bicycle-wheel dividing the area of the wheel into "triangles" (the outer edge of a bicycle wheel is of course round, but I hope you get the idea).
UPDATE: I found a small diagram on the web: