Hi Matt,
There is no special tool, and there are several ways to achieve the goal. It depends of what low poly terrain we are talking here.
You could use the displacement modifier with a heightmap texture at a grid primitive with a desired resolution. You could polygon model your terrain. Or you could sculpt it at a high divided grid primitive. And then reduce the size with the remesh modifier or by retopology.
Kind regards
Reiner
EDIT, ah forgot about the ANT Landscape addon. It is included in Blender and Bforartists. You just need to activate it. But it does basically nothing else than a displacement operation with random values.
EDIT2, shots are from Bforartists 2 development version. But should look somehow equal in Bforartists 1