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>
diff --git a/meta-google/recipes-google/networking/gbmc-bridge/-bmc-gbmcbr.network.in b/meta-google/recipes-google/networking/gbmc-bridge/-bmc-gbmcbr.network.in
index afea5cc..09ef620 100644
--- a/meta-google/recipes-google/networking/gbmc-bridge/-bmc-gbmcbr.network.in
+++ b/meta-google/recipes-google/networking/gbmc-bridge/-bmc-gbmcbr.network.in
@@ -7,5 +7,8 @@
 LLMNR=true
 MulticastDNS=true
 LinkLocalAddressing=ipv6
+IPv6PrefixDelegation=yes
 [IPv6AcceptRA]
 DHCPv6Client=false
+[IPv6PrefixDelegation]
+RouterLifetimeSec=30