cpu/esp*: esp_wifi used as default netdev for lwip

[gschorcht]

Contribution description

With this PR, esp-wifi is defined as default network device for pkg/lwip.

Normally, esp_now is used as the default network device for ESP modules. However, esp_now cannot be used together with pkg/lwip. Therefore, esp_wifi is used instead as the default network device if module lwip is enabled.

Testing procedure

Flash tests/lwip to any ESP32 and any ESP8266 board and ping the nodes from any machine in the LAN.

LWIP_IPV4=1 CFLAGS='-DESP_WIFI_SSID=\"ssid\" -DESP_WIFI_PASS=\"pass\"' \
make BOARD=esp32-wroom-32 -C tests/lwip flash term

LWIP_IPV4=1 CFLAGS='-DESP_WIFI_SSID=\"ssid\" -DESP_WIFI_PASS=\"pass\"' \
make BOARD=esp8266-esp-12x -C tests/lwip flash term

Issues/PRs references

[gschorcht]

@benpicco Thanks for setting the labels. I just forgot it because of the limited time I have on my holiday trip..

[benpicco]

Looks good.
I'd suggest moving that entire block to esp_common/Makefile.include though.
Duplicate code blocks like this will just get out-of sync eventually and there is nothing esp32/8266 specific here.

[gschorcht]

Looks good.
I'd suggest moving that entire block to esp_common/Makefile.include though.
Duplicate code blocks like this will just get out-of sync eventually and there is nothing esp32/8266 specific here.

Yeah. It's just an interim solution. There is already PR #12955 with complete code deduplication where it will be in esp_common/Makefile.include.

[gschorcht]

Thanks for reviewing and merging.