Add a kernel config for the Onion Omega

Small $25 IoT device, 400mhz Atheros cpu, Atheros WiFi and Ethernet
18 GPIOs, and support for Relay, Servo, and OLED expansion
https://onion.io/omega/

Reviewed by: adrian
Approved by: bapt (mentor)
Relnotes: yes
Sponsored by: ScaleEngine Inc.
Differential Revision: https://reviews.freebsd.org/D4188