rootdir: goldfish: Fix network route during emulation.

The net.eth0.gw system property is used by the ConnectivityService
to set the default route when it detects that 3G connectivity is
established. Because the property was undefined, the route was
unset which broke networking.

+ Format / document init.goldfish.sh

Change-Id: I1133cf6c093609300315cd0ea363c9e139b42521