base-files: migrate old UCI network bridge ports syntax

netifd supports more accurate "ports" option (instead of "ifname") for
years now. Relevant changes were even backported to OpenWrt 21.02. Add
uci-defaults script that translates config files using deprecated
syntax.

Identical commit f716c30241 ended up reverted back in 2021 by
80be798d4a due to breaking downgrades. With 19.07 support ended it
should no longer be an issue.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>

package/base-files/files/etc/uci-defaults/11_network-migrate-bridges

@@ -0,0 +1,23 @@
+. /lib/functions.sh
+
+migrate_ports() {
+	local config="$1"
+	local type ports ifname
+
+	config_get type "$config" type
+	[ "$type" != "bridge" ] && return
+
+	config_get ports "$config" ports
+	[ -n "$ports" ] && return
+
+	config_get ifname "$config" ifname
+	[ -z "$ifname" ] && return
+
+	for port in $ifname; do
+		uci add_list network.$config.ports="$port"
+	done
+	uci delete network.$config.ifname
+}
+
+config_load network
+config_foreach migrate_ports device