Hi, you can configure a standard keyboard to make EmulationStation not ask you for a controller on start-up, even if the keyboard not present. You can accomplish this in two ways (any of which only needed once):
Connect a standard USB keyboard and configure it in ES like any other controller.
If you are proficient with accesing the config files (SSH, Samba) and don't have a keyboard at hand, you can add the following snippet to /opt/retropie/configs/all/emulationstation/es_input.cfg to add a keyboard with standard mappings:
<inputConfig type="keyboard" deviceName="Keyboard" deviceGUID="-1">
<input name="pageup" type="key" id="113" value="1"/>
<input name="start" type="key" id="13" value="1"/>
<input name="down" type="key" id="1073741905" value="1"/>
<input name="pagedown" type="key" id="119" value="1"/>
<input name="right" type="key" id="1073741903" value="1"/>
<input name="select" type="key" id="1073742053" value="1"/>
<input name="left" type="key" id="1073741904" value="1"/>
<input name="up" type="key" id="1073741906" value="1"/>
<input name="a" type="key" id="115" value="1"/>
<input name="b" type="key" id="120" value="1"/>
<input name="x" type="key" id="97" value="1"/>
<input name="y" type="key" id="122" value="1"/>
</inputConfig>
This snippet has to be added between existing <inputList> and </inputList> tags.
An added benefit of configuring a keyboard is that if you don't have any controllers handy you can always plug a keyboard and control ES with it. Good luck!