host specific drivers can now be loaded separately

this ensures cleaner abstractions/code and will ensure that the produced
binary is as small as possible. a convenience package is provided to
easily load all hosts easily: "github.com/kidoman/embd/host/all"

host/generic/led.go

@@ -0,0 +1,113 @@
+// LED control support.
+
+package generic
+
+import (
+	"fmt"
+	"io/ioutil"
+	"os"
+	"strings"
+
+	"github.com/kidoman/embd"
+)
+
+type led struct {
+	id string
+
+	brightness *os.File
+
+	initialized bool
+}
+
+func NewLED(id string) embd.LED {
+	return &led{id: id}
+}
+
+func (l *led) init() error {
+	if l.initialized {
+		return nil
+	}
+
+	var err error
+	if l.brightness, err = l.brightnessFile(); err != nil {
+		return err
+	}
+
+	l.initialized = true
+
+	return nil
+}
+
+func (l *led) brightnessFilePath() string {
+	return fmt.Sprintf("/sys/class/leds/%v/brightness", l.id)
+}
+
+func (l *led) openFile(path string) (*os.File, error) {
+	return os.OpenFile(path, os.O_RDWR, os.ModeExclusive)
+}
+
+func (l *led) brightnessFile() (*os.File, error) {
+	return l.openFile(l.brightnessFilePath())
+}
+
+func (l *led) On() error {
+	if err := l.init(); err != nil {
+		return err
+	}
+
+	_, err := l.brightness.WriteString("1")
+	return err
+}
+
+func (l *led) Off() error {
+	if err := l.init(); err != nil {
+		return err
+	}
+
+	_, err := l.brightness.WriteString("0")
+	return err
+}
+
+func (l *led) isOn() (bool, error) {
+	l.brightness.Seek(0, 0)
+	bytes, err := ioutil.ReadAll(l.brightness)
+	if err != nil {
+		return false, err
+	}
+	str := string(bytes)
+	str = strings.TrimSpace(str)
+	if str == "1" {
+		return true, nil
+	}
+	return false, nil
+}
+
+func (l *led) Toggle() error {
+	if err := l.init(); err != nil {
+		return err
+	}
+
+	state, err := l.isOn()
+	if err != nil {
+		return err
+	}
+
+	if state {
+		return l.Off()
+	}
+	return l.On()
+}
+
+func (l *led) Close() error {
+	if !l.initialized {
+		return nil
+	}
+
+	if err := l.brightness.Close(); err != nil {
+		return err
+	}
+
+	l.initialized = false
+
+	return nil
+}