scripts: west_command: build: Support for conditional platform configs

extra_configs or extra_args can use additional filtering which was not
supported by the west build command. West is not aware of arch so that
filter type is not supported but platform (board) is known so it can
be used to apply expected configuration.

Signed-off-by: Krzysztof Chruściński <krzysztof.chruscinski@nordicsemi.no>