Tweak the logic for finding a file to set a config setting in.
Tweak the logic for finding a file to set a config setting in.

This fixes a bug where a low-priority readonly config file would
prevent those settings being changed.

It also solves the problem of all settings going into /etc/piaware.conf
if it exists (e.g. from a piaware package upgrade from 2.x), even if
/boot/piaware-config.txt also exists.

The new logic prefers to change settings in place where possible, and
when not possible it will prefer to put new settings in the highest
priority writable file (i.e. /boot/piaware-config.txt if it exists)

flightaware/piaware,flightaware/piaware