RPi 3B+ WiFi Firmware
Raspberry Pi 3B+ WiFi not working.
Missing firmware is available in recent Buildroot.
RF Kill is also causing issues by blocking wlan0. Connman intercepts /dev/rfkill for some reason to make this even more complex.
WiFi is working well on 3B/3B+ in other recent RPi distributions using simple wpa_supplicant based solutions.