cmake: modules: west: allow custom MIN_WEST_VERSION

Different users of the Zephyr CMake package may have different minimum
required versions of west. One important in-tree example is the
documentation, which must either be built with the latest version of
west, or with no west installed at all.

Make the MIN_WEST_VERSION variable configurable to support use cases
like this.

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