base-files: unify get_dt_led helper function

Lantiq and IPQ806X (which includes IPQ40XX) both define the
same custom function {ipq806x|lantiq}_get_dt_led.

This patch moves the function into the base-file package at
lib/functions/leds.sh to make it more accessible for other
targets as well.

Cc: Mathias Kresin <dev@kresin.me>
Cc: John Crispin <john@phrozen.org>
Cc: Hannu Nyman <hannu.nyman@iki.fi>
Signed-off-by: Christian Lamparter <chunkeey@gmail.com>

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

@@ -1,6 +1,18 @@
 #!/bin/sh
 # Copyright (C) 2013 OpenWrt.org
 
+get_dt_led() {
+	local label
+	local ledpath
+	local basepath="/proc/device-tree"
+	local nodepath="$basepath/aliases/led-$1"
+
+	[ -f "$nodepath" ] && ledpath=$(cat "$nodepath")
+	[ -n "$ledpath" ] && label=$(cat "$basepath$ledpath/label")
+
+	echo "$label"
+}
+
 led_set_attr() {
 	[ -f "/sys/class/leds/$1/$2" ] && echo "$3" > "/sys/class/leds/$1/$2"
 }