$fields['my_field'] = BaseFieldDefinition::create("list_string")
      ->setSetting('allowed_values', $$my_options)
      ->setLabel('My Field')
      ->setRequired(FALSE)
      ->setCardinality(-1)
      ->setDisplayOptions('form', array(
        'type' => 'options_buttons',
        'weight' => 5
      ))
      ->setDisplayConfigurable('form', TRUE)
      ->setDisplayConfigurable('view', TRUE);