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

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

src/machine/machine_dummy.go

@@ -25,3 +25,7 @@ func (p GPIO) Configure(config GPIOConfig) {
 
 func (p GPIO) Set(value bool) {
 }
+
+func (p GPIO) Get() bool {
+	return false
+}