common: Use IPv4/6 default configuration on all interfaces

The comment in the script has the story.

board/common/rootfs/etc/finit.d/10-infix.conf

@@ -1 +1,2 @@
+run [S] /lib/infix/sysctl-sync-ip-conf --
 run [S] /lib/infix/nameif -- Probing network interfaces

board/common/rootfs/lib/infix/sysctl-sync-ip-conf

@@ -0,0 +1,14 @@
+#!/bin/sh
+
+# When changing default IPv4/6 settings, this does not apply to
+# existing interfaces. This syncs the default settings to all known
+# interfaces at boot, such that physical interfaces will start from
+# the same point as virtual ones.
+
+sysctl net.ipv4.conf.default  >/tmp/sysctl-ip-default
+sysctl net.ipv6.conf.default >>/tmp/sysctl-ip-default
+
+for iface in $(ip -j link show | jq -r .[].ifname); do
+    sed -e "s/.default./.${iface}./g" /tmp/sysctl-ip-default \
+	| sysctl -p -
+done