scripts: add list_boards.py

This script is essentially a Python rewrite of the CMake code we're
using to print boards in cmake/boards.cmake, plus some extra features.

Having this in Python will simplify some later adjustments to our
'usage' build system target in ways that will make its output easier
to read, while simultaneously making 'west boards' more useful.

Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>