meta-google: gbmc-bridge: Rework IP address persistence

This consolidates all of the mechanisms which write out a persistent IP
into a single place. It also transitions to writing a very simple
persistent file instead of systemd style network units.

Change-Id: Ib99d7646178d2c5383cf23b09248bf24544c1d9e
Signed-off-by: William A. Kennington III <wak@google.com>