Fix networking.service waiting for udevadm settle (#2295)

There was a fix to speed up initialization when networking used init.d
but it did not carry over to systemd networking.service. This fix will
apply the same change on the systemd service.

The result is much less time spent being blocked in networking.service.

Author: zzhiyuan

files/build_templates/sonic_debian_extension.j2

@@ -259,9 +259,10 @@ sudo dpkg --root=$FILESYSTEM_ROOT -P {{ debname }}
 
 sudo rm -f $FILESYSTEM_ROOT/usr/sbin/policy-rc.d
 
-## Revise /etc/init.d/networking for Arista switches
+## Revise /etc/init.d/networking and /lib/systemd/system/networking.service for Arista switches
 if [ "$image_type" = "aboot" ]; then
     sudo sed -i 's/udevadm settle/udevadm settle -E \/sys\/class\/net\/eth0/' $FILESYSTEM_ROOT/etc/init.d/networking
+    sudo sed -i 's/udevadm settle/udevadm settle -E \/sys\/class\/net\/eth0/' $FILESYSTEM_ROOT/lib/systemd/system/networking.service
 fi
 
 ## copy platform rc.local