base-files: add leds migration

Currently leds migration scripts in ar71xx and lantiq share a lot of
logic and introducing leds migration to another target would mean
copying this code, again. Therefore add common logic to library in
base-files package.

Suggested-by: Petr Štetiar <ynezz@true.cz>
Signed-off-by: Petr Štetiar <ynezz@true.cz>
Signed-off-by: Tomasz Maciej Nowak <tomek_n@o2.pl>

package/base-files/files/lib/functions/migrations.sh

@@ -0,0 +1,39 @@
+#!/bin/sh
+
+. /lib/functions.sh
+
+migrate_led_sysfs() {
+	local cfg="$1"; shift
+	local tuples="$@"
+	local sysfs
+	local name
+
+	config_get sysfs ${cfg} sysfs
+	config_get name ${cfg} name
+
+	[ -z "${sysfs}" ] && return
+
+	for tuple in ${tuples}; do
+		local old=${tuple%=*}
+		local new=${tuple#*=}
+		local new_sysfs
+
+		new_sysfs=$(echo ${sysfs} | sed "s/${old}/${new}/")
+
+		[ "${new_sysfs}" = "${sysfs}" ] && continue
+
+		uci set system.${cfg}.sysfs="${new_sysfs}"
+
+		logger -t led-migration "sysfs option of LED \"${name}\" updated to ${new_sysfs}"
+	done;
+}
+
+migrate_leds() {
+	config_load system
+	config_foreach migrate_led_sysfs led "$@"
+}
+
+migrations_apply() {
+	local realm="$1"
+	[ -n "$(uci changes ${realm})" ] && uci -q commit ${realm}
+}