scripts: improve west build's board handling

- Respect the BOARD environment setting.
- Don't require --force if the board can't be figured out: it might be
  set in CMakeLists.txt, for example. Instead, downgrade to a warning
  which can be disabled with "west config build.board_warn false".
- Add a build.board configuration option used as a final BOARD fallback
  after CACHED_BOARD (in the CMake cache), --board (command line), and
  BOARD (environment).
- Keep the config docs up to date.

Signed-off-by: Marti Bolivar <marti.bolivar@nordicsemi.no>