Get wifi strength from connman/iwd/etc.
The current way of getting the wifi strength is polling the hardcoded wlan0
via WEXT, and while this is simple it is wrong on many levels: Hardcoding device names is bad for stability and portability, WEXT has been deprecated for a very long time now, and polling is unnecessary when we can get notified over DBus (it might not save power as the polling interval is low, but it will decrease latency significantly).
The reason why this is not done yet is that connmans DBus-interface seems to be very much a PITA.
EDIT: Of course we might also want to support iwd or some other alternative (NetworkManager? Maybe not used much in embedded systems). The current platform (Pi zero) uses connman because iwd seems not to work with brcmfmac, but other platforms may be more lucky.