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/bbb/analogpin.go

@@ -0,0 +1,104 @@
+// Analog I/O support on the BBB.
+
+package bbb
+
+import (
+	"fmt"
+	"io/ioutil"
+	"os"
+	"strconv"
+	"strings"
+
+	"github.com/kidoman/embd"
+)
+
+type analogPin struct {
+	id string
+	n  int
+
+	drv embd.GPIODriver
+
+	val *os.File
+
+	initialized bool
+}
+
+func newAnalogPin(pd *embd.PinDesc, drv embd.GPIODriver) embd.AnalogPin {
+	return &analogPin{id: pd.ID, n: pd.AnalogLogical, drv: drv}
+}
+
+func (p *analogPin) N() int {
+	return p.n
+}
+
+func (p *analogPin) init() error {
+	if p.initialized {
+		return nil
+	}
+
+	var err error
+	if err = p.ensureEnabled(); err != nil {
+		return err
+	}
+	if p.val, err = p.valueFile(); err != nil {
+		return err
+	}
+
+	p.initialized = true
+
+	return nil
+}
+
+func (p *analogPin) ensureEnabled() error {
+	return ensureFeatureEnabled("cape-bone-iio")
+}
+
+func (p *analogPin) valueFilePath() (string, error) {
+	pattern := fmt.Sprintf("/sys/devices/ocp.*/helper.*/AIN%v", p.n)
+	return embd.FindFirstMatchingFile(pattern)
+}
+
+func (p *analogPin) openFile(path string) (*os.File, error) {
+	return os.OpenFile(path, os.O_RDONLY, os.ModeExclusive)
+}
+
+func (p *analogPin) valueFile() (*os.File, error) {
+	path, err := p.valueFilePath()
+	if err != nil {
+		return nil, err
+	}
+	return p.openFile(path)
+}
+
+func (p *analogPin) Read() (int, error) {
+	if err := p.init(); err != nil {
+		return 0, err
+	}
+
+	p.val.Seek(0, 0)
+	bytes, err := ioutil.ReadAll(p.val)
+	if err != nil {
+		return 0, err
+	}
+	str := string(bytes)
+	str = strings.TrimSpace(str)
+	return strconv.Atoi(str)
+}
+
+func (p *analogPin) Close() error {
+	if err := p.drv.Unregister(p.id); err != nil {
+		return err
+	}
+
+	if !p.initialized {
+		return nil
+	}
+
+	if err := p.val.Close(); err != nil {
+		return err
+	}
+
+	p.initialized = false
+
+	return nil
+}