avr: implement Get() function on AVR, and leave stubs for NRF and dummy machines

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

src/examples/button/button.go

@@ -0,0 +1,28 @@
+package main
+
+import (
+	"machine"
+	"runtime"
+)
+
+// This example assumes that the button is connected to pin 8. Change the value
+// below to use a different pin.
+const buttonPin = 8
+
+func main() {
+	led := machine.GPIO{machine.LED}
+	led.Configure(machine.GPIOConfig{Mode: machine.GPIO_OUTPUT})
+
+	button := machine.GPIO{buttonPin}
+	button.Configure(machine.GPIOConfig{Mode: machine.GPIO_INPUT})
+
+	for {
+		if button.Get() {
+			led.Low()
+		} else {
+			led.High()
+		}
+
+		runtime.Sleep(runtime.Millisecond * 10)
+	}
+}