cpu/esp: add feature arch_esp for all ESP SoCs
cpu/esp: add feature arch_esp for all ESP SoCs

There are common features that can be used by ESP32 as well as ESP8266, for example the `esp_wifi` module. To be able to test for any ESP SoC, feature `arch_esp` is introduced.
OlegHahm/RIOT,OlegHahm/RIOT,authmillenon/RIOT,miri64/RIOT,jasonatran/RIOT,RIOT-OS/RIOT,kYc0o/RIOT,authmillenon/RIOT,RIOT-OS/RIOT,miri64/RIOT,kaspar030/RIOT,kYc0o/RIOT,OlegHahm/RIOT,miri64/RIOT,kYc0o/RIOT,OTAkeys/RIOT,RIOT-OS/RIOT,jasonatran/RIOT,kYc0o/RIOT,authmillenon/RIOT,jasonatran/RIOT,jasonatran/RIOT,ant9000/RIOT,jasonatran/RIOT,OTAkeys/RIOT,miri64/RIOT,kaspar030/RIOT,RIOT-OS/RIOT,ant9000/RIOT,RIOT-OS/RIOT,ant9000/RIOT,authmillenon/RIOT,ant9000/RIOT,kaspar030/RIOT,OTAkeys/RIOT,kaspar030/RIOT,kYc0o/RIOT,authmillenon/RIOT,ant9000/RIOT,OTAkeys/RIOT,authmillenon/RIOT,OTAkeys/RIOT,miri64/RIOT,kaspar030/RIOT,OlegHahm/RIOT,OlegHahm/RIOT