base-files: diag.sh: Make it more generic towards DTS so it could be reused

I wanted to add status LEDs support to my imx6 based board and have
found out, that I could use diag.sh script found in ramips platform,
which seems to be also shared in a few other platforms:

 4801276bc2078c5bcf03003c831e3b0a target/linux/ramips/base-files/etc/diag.sh
 4801276bc2078c5bcf03003c831e3b0a target/linux/ipq40xx/base-files/etc/diag.sh
 4801276bc2078c5bcf03003c831e3b0a target/linux/ath79/base-files/etc/diag.sh

So I've extended the base diag.sh in a way, that if it detects any of
the DTS LED aliases, then it would use the generic DTS set_led_state
code.

Signed-off-by: Petr Štetiar <ynezz@true.cz>

package/base-files/files/etc/diag.sh

@@ -1,4 +1,50 @@
 #!/bin/sh
-# Copyright (C) 2006-2009 OpenWrt.org
+# Copyright (C) 2006-2019 OpenWrt.org
 
-set_state() { :; }
+. /lib/functions/leds.sh
+
+boot="$(get_dt_led boot)"
+failsafe="$(get_dt_led failsafe)"
+running="$(get_dt_led running)"
+upgrade="$(get_dt_led upgrade)"
+
+set_led_state() {
+	status_led="$boot"
+
+	case "$1" in
+	preinit)
+		status_led_blink_preinit
+		;;
+	failsafe)
+		status_led_off
+		[ -n "$running" ] && {
+			status_led="$running"
+			status_led_off
+		}
+		status_led="$failsafe"
+		status_led_blink_failsafe
+		;;
+	preinit_regular)
+		status_led_blink_preinit_regular
+		;;
+	upgrade)
+		[ -n "$running" ] && {
+			status_led="$running"
+			status_led_off
+		}
+		status_led="$upgrade"
+		status_led_blink_preinit_regular
+		;;
+	done)
+		status_led_off
+		[ -n "$running" ] && {
+			status_led="$running"
+			status_led_on
+		}
+		;;
+	esac
+}
+
+set_state() {
+	[ -n "$boot" -o -n "$failsafe" -o -n "$running" -o -n "$upgrade" ] && set_led_state "$1"
+}