esp_now: use NETDEV_TYPE_ESP_NOW as device type

[miri64]

Contribution description

This device should get its own device type similar to the cc110x as it is its own protocol. Using NETDEV_TYPE_RAW might lead to problems in the long run if other devices use the same device type but have different addressing schemes.

Testing procedure

Applications using IPv6 ESP-Now should still work as before (can't test since I don't have access to these kind of devices).

Issues/PRs references

None so far, but goes towards an effort to improve device-type-based address handling for gnrc_netif.

[gschorcht]

Tested it with two different ESP32 boards

USEMODULE=esp_now make flash -C examples/gnrc_networking BOARD=esp32-wroom-32 port=/dev/ttyUSB0

and

USEMODULE=esp_now make flash -C examples/gnrc_networking BOARD=esp32-mh-et-live-minikit flash PORT=/dev/ttyUSB1

[gschorcht]

Thanks, I have to this also for PR #9917

[miri64]

Thanks you for the review :-)