ODROID-U3 update (#7)

Migration from Arch Wiki

ODROID-U3.md

@@ -11,4 +11,26 @@ ctl.!default {
    type hw
    card 0
 }
-```
+```
+## LEDs
+ODROIDs have dual LEDs: a red power LED which is always on if power is
+supplied, and a blue LED which can be configured.
+
+By default, the blue LED is a heartbeat LED, which flashes when the kernel is
+running. This can be configured in `/sys/class/leds/led1/trigger`.
+
+To list available triggers:
+
+    # cat /sys/class/leds/blue:heartbeat/trigger
+
+Replace TRIGGER with one of the available triggers. This setting will apply
+instantly, but be lost upon reboot.
+
+    # echo TRIGGER > /sys/class/leds/blue:heartbeat/trigger
+
+To create a persistent configuration, edit or create
+`/etc/tmpfiles.d/leds.conf`:
+
+    w /sys/class/leds/blue:heartbeat/trigger - - - - TRIGGER
+
+