add support for an override config directory

add support for an override config directory

This can be used to override specific config files at runtime without
touching them on primary storage. It is used by default for both reading
and updating.
The primary use case for this is adding a config management system which
generates and manages uci files at run time without overwriting existing
config files on flash.

Signed-off-by: Felix Fietkau <nbd@nbd.name>