code: function getBuildPath(rootPath, buildPath) {
      let result = buildPath || './build';
  
      if (!path.isAbsolute(result)) {
          result = path.resolve(rootPath, result);
      }
  
      return result;
  }
docs: 获得 mock server 的构建之后的根目录绝对路径
  @param {String} rootPath 项目根目录
  @param {String} buildPath
  @returns {String}