meta-google: gbmc-bridge: Conditionally advertise default route

We only want to advertise that we have a default route if we have an
external (non-bridge) interface that has a default route. This
disambiguates routes on the internal machine network to avoid
blackholes.

Change-Id: I4059ddf0f74a0b15ca3f93f096b79415a851d3f7
Signed-off-by: William A. Kennington III <wak@google.com>