at: parse config file

The AT commands list for each stage is now configured per-device, as 
well as the UART port to be used.

As this list can be changed by the user, there is no more need for a 
special GNSS-related option

src/manager.h

@@ -30,7 +30,6 @@ enum EG25State {
 struct EG25Manager {
     GMainLoop *loop;
     guint reset_timer;
-    gboolean manage_gnss;
     gboolean use_libusb;
     guint usb_vid;
     guint usb_pid;