When usb-moded causes network interfaces to be brought up/down, firewall
rules should be adjusted accordingly. However currently there is no way
for connman to know which interface changes might be related to for
example developer mode activation.

Expose dynamic usb mode configuration (attributes relevant to connman)
on D-Bus.

Broadcast a signal whenever target mode changes (mode transition begins,
or fallback mode gets activated due to mode transition errors).

Also provide a method call for querying the current target configuration.
Signed-off-by: Simo Piiroinen <simo.piiroinen@jollamobile.com>