irled: implement option to read irled state (themactep#166)

Summary:
Unlike motors and ircut, the irled script lacks a status reporting
option. This patch implements a read option.

Although the underlying gpio script has a read function, this
patch bypasses that abstraction to read the sys tree directly.
This is a measured and intentional tradeoff. Output from the
gpio script is decorated with unwanted formatting codes and
superfluous data fields, which can't be disabled. As the
intended application for this output is another program, clean
and simple output is preferred.

This will be plumbed through to a web api in a future patch,
pending reflection on the most natural way to mesh that feature
with the existing APIs.

The targeted application is an unpublished Android mobile
application for viewing and controlling thingino pan-tilt
cameras on a local network, which will be open-sourced when
ready.

A future submission may implement a clean output mode for the
gpio script, and modify this call site to use that abstraction
layer.

Testing:
This was tested from the ssh command line on a Cinnado D1 and
a WUUK Y0510. In addition to verifying the correct readings,
the state changes of the IR led were also verified by
inspection.

```
test # irled on
test # irled read
1
test # irled off
test # irled read
0
test # irled read ir850
0
test # irled on ir850
test # irled read ir850
1
test #
```

overlay/lower/usr/sbin/irled

@@ -32,6 +32,9 @@ case "$mode" in
 	off | 0)
 		gpio clear $pin > /dev/null
 		;;
+	read)
+		cat /sys/class/gpio/gpio${pin}/value
+		;;
 	~ | toggle)
 		gpio toggle $pin > /dev/null
 		;;