gpsd: migrate option device to a list devices

gpsd: migrate option device to a list devices

More than one device can be added to the gpsd at startup. Currently the gpsd
service start script in OpenWrt treats this as an option with only one value.
To allow multiple devices to be specified, the uci 'option device' must be
removed and a new uci 'list devices' is added.

This change means that several devices can be specified at gpsd start with
the new uci 'list devices' configuration.

Running configurations in the field are migrated by a migration script.

Signed-off-by: Florian Eckert <fe@dev.tdt.de>