up2: useful constant values to access the built-in LEDs

Signed-off-by: Ron Evans <ron@hybridgroup.com>

examples/up2_leds.go

@@ -14,10 +14,10 @@ import (
 
 func main() {
 	b := up2.NewAdaptor()
-	red := gpio.NewLedDriver(b, "red")
-	blue := gpio.NewLedDriver(b, "blue")
-	green := gpio.NewLedDriver(b, "green")
-	yellow := gpio.NewLedDriver(b, "yellow")
+	red := gpio.NewLedDriver(b, up2.LEDRed)
+	blue := gpio.NewLedDriver(b, up2.LEDBlue)
+	green := gpio.NewLedDriver(b, up2.LEDGreen)
+	yellow := gpio.NewLedDriver(b, up2.LEDYellow)
 
 	work := func() {
 		red.Off()

platforms/upboard/up2/README.md

@@ -80,6 +80,13 @@ r := up2.NewAdaptor()
 led := gpio.NewLedDriver(r, "13")
 ```
 
+You can also use the values `up2.LEDRed`, `up2.LEDBlue`, `up2.LEDGreen`, and `up2.LEDYellow` as pin reference to access the 4 built-in LEDs. For example:
+
+```go
+r := up2.NewAdaptor()
+led := gpio.NewLedDriver(r, up2.LEDRed)
+```
+
 ## How to Connect
 
 ### Compiling

platforms/upboard/up2/adaptor.go

@@ -14,6 +14,20 @@ import (
 	"gobot.io/x/gobot/sysfs"
 )
 
+const (
+	// LEDRed is the built-in red LED.
+	LEDRed = "red"
+
+	// LEDBlue is the built-in blue LED.
+	LEDBlue = "blue"
+
+	// LEDGreen is the built-in green LED.
+	LEDGreen = "green"
+
+	// LEDYellow is the built-in yellow LED.
+	LEDYellow = "yellow"
+)
+
 type sysfsPin struct {
 	pin    int
 	pwmPin int
@@ -110,7 +124,7 @@ func (c *Adaptor) DigitalRead(pin string) (val int, err error) {
 // DigitalWrite writes digital value to the specified pin.
 func (c *Adaptor) DigitalWrite(pin string, val byte) (err error) {
 	// is it one of the built-in LEDs?
-	if pin == "red" || pin == "blue" || pin == "green" || pin == "yellow" {
+	if pin == LEDRed || pin == LEDBlue || pin == LEDGreen || pin == LEDYellow {
 		pinPath := fmt.Sprintf(c.ledPath, pin)
 		fi, e := sysfs.OpenFile(pinPath, os.O_WRONLY|os.O_APPEND, 0666)
 		defer fi.Close()