meta-google: gbmc-bridge: Terminate DHCP after reachable

We don't want to continue running DHCP after a machine is ready to serve
traffic. We will only run the DHCP process for a short amount of time if
we can determine that the BMC can reach network services because it was
already correctly configured.

Change-Id: Ie79e0582718af88a4a170b2a77beebfbadd5d0af
Signed-off-by: William A. Kennington III <wak@google.com>