Here is a newer version of the ESP firmware that now utilises the wifi_send_pkt_freedom() function to send beacon frames.
This method of generation allows for tighter control of beacon frame rates and increases security as it runs in a passive client mode and only generates beacon frames.
Note that the Node firmware has also been updated to support this new version. You will need to use the latest Node firmware (0.1.6) with this firmware. Additionally, the old ESP firmware will not work with the latest Node firmware.