scripts: add build.generator config option for west build

This can be used to override the default CMake generator
permanently. Its values are the same as those acceptable to cmake's -G
option.

Signed-off-by: Marti Bolivar <marti.bolivar@nordicsemi.no>
diff --git a/scripts/west_commands/build.py b/scripts/west_commands/build.py
index 798f7b8..930e61b 100644
--- a/scripts/west_commands/build.py
+++ b/scripts/west_commands/build.py
@@ -403,7 +403,8 @@
         # west build -- -DOVERLAY_CONFIG=relative-path.conf
         final_cmake_args = ['-B{}'.format(self.build_dir),
                             '-S{}'.format(self.source_dir),
-                            '-G{}'.format(DEFAULT_CMAKE_GENERATOR)]
+                            '-G{}'.format(config_get('generator',
+                                                     DEFAULT_CMAKE_GENERATOR))]
         if cmake_opts:
             final_cmake_args.extend(cmake_opts)
         run_cmake(final_cmake_args)