Magento2.1 Category custom attribute dropdown

Add options tag for creating select options. In your case this should be

<field name="roflcopter">
    <argument name="data" xsi:type="array">
        <item name="options" xsi:type="object">Magento\Catalog\Model\Category\Attribute\Source\Page</item>
        <item name="config" xsi:type="array">
            <item name="sortOrder" xsi:type="number">70</item>
            <item name="dataType" xsi:type="string">string</item>
            <item name="formElement" xsi:type="string">select</item>
            <item name="label" xsi:type="string" translate="true">Roflcopter</item>
        </item>
    </argument>
</field>

I have done in my case. I have custom options ex. L1, L2 and L3. I need to get them on custom attribute as values. So I was created a source file in module - vendor\module\Model\Config\Source\Options.php

this file contain the small code to create the options, Here you can follow the code

 <?php
    /**
     * Copyright © 2013-2017 Magento, Inc. All rights reserved.
     * See COPYING.txt for license details.
     */
    namespace Vendor\module\Model\Config\Source;
    /**
     * Catalog category landing page attribute source
     *
     * @author      Magento Core Team <[email protected]>
     */
    class Options extends \Magento\Eav\Model\Entity\Attribute\Source\AbstractSource
    {
        /**
         * {@inheritdoc}
         * @codeCoverageIgnore
         */
        public function getAllOptions()
        {
            if (!$this->_options) {
                $this->_options = [
                    ['value' => 'l1', 'label' => __('L1')],
                    ['value' => 'l2', 'label' => __('L2')],
                    ['value' => 'l3', 'label' => __('L3')],
                ];
            }
            return $this->_options;
        }
          /**
         * Get options in "key-value" format
         *
         * @return array
         */
        public function toArray()
        {
            return [
                'l1' => __('L1'),
                'l2' => __('L2'),
                'L3' => __('L3'),
                ];
        }

    }

then after in your installdata.php you have to call this as source

$eavSetup->addAttribute(
            Category::ENTITY,
            'category_level_rendering',
            [
                'type' => 'varchar',
                'backend' => '',
                'frontend' => '',
                'label' => 'Category Level rendering',
                'input' => 'select',
                'required' => false,
                'sort_order' => 100,
                'source' => '',
                'visible'  => true,
                'source' => 'vendor\module\Model\Config\Source\Options',
                'default'  => '0',
                'global' => \Magento\Eav\Model\Entity\Attribute\ScopedAttributeInterface::SCOPE_STORE,
                'group' => 'General Information',
                'used_in_product_listing' => true,
             ]
        );

Then also add the line in xml file

<field name="category_level_rendering">
                <argument name="data" xsi:type="array">
/*Here is the code added to get the options on dropdown*/
<item name="options" xsi:type="object">Vendor\module\Model\Config\Source\Options</item>
                    <item name="config" xsi:type="array">
                        <item name="sortOrder" xsi:type="number">10</item>
                        <item name="dataType" xsi:type="string">string</item>
                        <item name="formElement" xsi:type="string">select</item>
                        <item name="label" xsi:type="string" translate="true">Category Level Rendering</item>
                    </item>
                </argument>
            </field>

Save it, flush cache and please check.

Hopefully it helps to you.

Please give me reply if it works to you.