board/common: fix bogus syslog warnings

When syncing interface defaults at boot, some settings are read-only and
some are write-only.  Filter these out so sysctl does not log any bogus
Permission Denied messages.

Signed-off-by: Joachim Wiberg <troglobit@gmail.com>

board/common/rootfs/libexec/infix/init.d/10-sysctl-sync-ip-conf

@@ -5,12 +5,24 @@
 # interfaces at boot, such that physical interfaces will start from
 # the same point as virtual ones.
 tmp=$(mktemp)
+fil=$(mktemp)
 
-sysctl net.ipv4.conf.default  >$tmp
-sysctl net.ipv6.conf.default >>$tmp
+# Ignore unreadable entries, like net.ipv6.conf.default.stable_secret
+sysctl net.ipv4.conf.default  >"$tmp" 2>/dev/null
+sysctl net.ipv6.conf.default >>"$tmp" 2>/dev/null
+
+# Filter out read-only entries like net.ipv4.conf.default.mc_forwarding
+# to prevent misleading error messages in syslog
+while IFS= read -r line; do
+    entry=$(echo "$line" | awk '{print $1}')
+    path="/proc/sys/$(echo "$entry" | tr . /)"
+    if [ -w "$path" ]; then
+        echo "$line" >> "$fil"
+    fi
+done < "$tmp"
 
 for iface in $(ip -j link show | jq -r .[].ifname); do
-    sed -e "s/.default./.${iface}./g" $tmp | sysctl -q -p -
+    sed -e "s/.default./.${iface}./g" "$fil" | sysctl -q -p -
 done
 
-rm $tmp
+rm "$tmp" "$fil"