gpio: adding interrupt

this is inspired by Dave Cheney's gpio library and his work on EPOLL

samples/gpiointerrupt.go

@@ -0,0 +1,42 @@
+// +build ignore
+
+package main
+
+import (
+	"flag"
+	"fmt"
+
+	"github.com/kidoman/embd"
+
+	_ "github.com/kidoman/embd/host/all"
+)
+
+func main() {
+	flag.Parse()
+
+	if err := embd.InitGPIO(); err != nil {
+		panic(err)
+	}
+	defer embd.CloseGPIO()
+
+	btn, err := embd.NewDigitalPin(10)
+	if err != nil {
+		panic(err)
+	}
+	defer btn.Close()
+
+	if err := btn.SetDirection(embd.In); err != nil {
+		panic(err)
+	}
+	btn.ActiveLow(false)
+
+	quit := make(chan interface{})
+	err = btn.Watch(embd.EdgeFalling, func(btn embd.DigitalPin) {
+		quit <- btn
+	})
+	if err != nil {
+		panic(err)
+	}
+
+	fmt.Printf("Button %v was pressed.\n", <-quit)
+}