cpu/esp32: defines esp_now as default netdev
cpu/esp32: defines esp_now as default netdev

If the user or the board definition doesn't enable `esp_wifi` or `esp_eth`, `esp_now` is defined as default netdev.
fixup! cpu/esp32: defines esp_now as default netdev

smlng/RIOT,OlegHahm/RIOT,smlng/RIOT,miri64/RIOT,authmillenon/RIOT,miri64/RIOT,RIOT-OS/RIOT,authmillenon/RIOT,toonst/RIOT,aeneby/RIOT,miri64/RIOT,jasonatran/RIOT,ant9000/RIOT,authmillenon/RIOT,kYc0o/RIOT,kYc0o/RIOT,basilfx/RIOT,jasonatran/RIOT,aeneby/RIOT,ant9000/RIOT,josephnoir/RIOT,OTAkeys/RIOT,basilfx/RIOT,toonst/RIOT,kaspar030/RIOT,kYc0o/RIOT,mtausig/RIOT,yogo1212/RIOT,OlegHahm/RIOT,kaspar030/RIOT,ant9000/RIOT,x3ro/RIOT,RIOT-OS/RIOT,josephnoir/RIOT,aeneby/RIOT,ant9000/RIOT,OTAkeys/RIOT,x3ro/RIOT,basilfx/RIOT,kaspar030/RIOT,mtausig/RIOT,yogo1212/RIOT,yogo1212/RIOT,authmillenon/RIOT,mtausig/RIOT,OTAkeys/RIOT,yogo1212/RIOT,OlegHahm/RIOT,smlng/RIOT,aeneby/RIOT,mtausig/RIOT,x3ro/RIOT,kaspar030/RIOT,RIOT-OS/RIOT,authmillenon/RIOT,kaspar030/RIOT,x3ro/RIOT,jasonatran/RIOT,aeneby/RIOT,miri64/RIOT,josephnoir/RIOT,kYc0o/RIOT,toonst/RIOT,OTAkeys/RIOT,kYc0o/RIOT,smlng/RIOT,josephnoir/RIOT,smlng/RIOT,x3ro/RIOT,josephnoir/RIOT,authmillenon/RIOT,miri64/RIOT,RIOT-OS/RIOT,mtausig/RIOT,jasonatran/RIOT,OTAkeys/RIOT,yogo1212/RIOT,OlegHahm/RIOT,basilfx/RIOT,toonst/RIOT,OlegHahm/RIOT,jasonatran/RIOT,toonst/RIOT,basilfx/RIOT,ant9000/RIOT,yogo1212/RIOT,RIOT-OS/RIOT