This adds an example configuration to use the gnrc_border_router to route
between a regular WiFi network and an esp_now network.
All you need is an esp based board (esp8266, esp32).
Configure the access data for your WiFi network in `Makefile.esp.conf`,
then run
make BOARD=esp32-wroom-32 USE_ETHOS=0 USE_ESP=1 flash term
to turn a e.g. `esp32-wroom-32` into a boarder router.
You can use other esp based boards as nodes in the network by flashing
e.g. the `gnrc_networking` example.
Be sure to add `CFLAGS += -DGNRC_IPV6_NIB_CONF_SLAAC=1` as otherwise
your esp_now node will not receive a link-local IP address.
