РАБОТАЕТ
app.use(async (req, res, next) => {
  res.locals.boards = await Board.find();
  next();
});


НЕ РАБОТАЕТ
app.use((req, res, next) => {
  Board.find()
    .then(result => {
      res.locals.boards = result;
    })
    .catch(err => console.log(err));
  next();
});