Added tests and ifx for board options available only under particular OS

See: https://github.com/arduino/arduino-cli/pull/67#issuecomment-429919298

quote:
There are some options without label (did not parsed well) for upload speed on different OS:

Option:     Upload Speed                             UploadSpeed
            115200                                 ✔ UploadSpeed=115200
            9600                                     UploadSpeed=9600
            57600                                    UploadSpeed=57600
            230400                                   UploadSpeed=230400
                                                     UploadSpeed=256000
            460800                                   UploadSpeed=460800
                                                     UploadSpeed=512000
            921600                                   UploadSpeed=921600

Cause boards.txt contains:

generic.menu.UploadSpeed.115200=115200
generic.menu.UploadSpeed.115200.upload.speed=115200
generic.menu.UploadSpeed.9600=9600
generic.menu.UploadSpeed.9600.upload.speed=9600
generic.menu.UploadSpeed.57600=57600
generic.menu.UploadSpeed.57600.upload.speed=57600
generic.menu.UploadSpeed.230400.linux=230400
generic.menu.UploadSpeed.230400.macosx=230400
generic.menu.UploadSpeed.230400.upload.speed=230400
generic.menu.UploadSpeed.256000.windows=256000
generic.menu.UploadSpeed.256000.upload.speed=256000
generic.menu.UploadSpeed.460800.linux=460800
generic.menu.UploadSpeed.460800.macosx=460800
generic.menu.UploadSpeed.460800.upload.speed=460800
generic.menu.UploadSpeed.512000.windows=512000
generic.menu.UploadSpeed.512000.upload.speed=512000
generic.menu.UploadSpeed.921600=921600
generic.menu.UploadSpeed.921600.upload.speed=921600

arduino/cores/board.go

@@ -78,7 +78,9 @@ func (b *Board) GetConfigOptionValues(option string) *properties.Map {
 	res := properties.NewMap()
 	menu := b.Properties.SubTree("menu").SubTree(option)
 	for _, value := range menu.FirstLevelKeys() {
-		res.Set(value, menu.Get(value))
+		if label, ok := menu.GetOk(value); ok {
+			res.Set(value, label)
+		}
 	}
 	return res
 }

arduino/cores/packagemanager/package_manager_test.go

@@ -22,6 +22,7 @@ import (
 
 	"github.com/arduino/arduino-cli/arduino/cores/packagemanager"
 	"github.com/arduino/go-paths-helper"
+	"github.com/arduino/go-properties-orderedmap"
 	"github.com/stretchr/testify/require"
 )
 
@@ -43,3 +44,44 @@ func TestFindBoardWithFQBN(t *testing.T) {
 	require.NotNil(t, board)
 	require.Equal(t, board.Name(), "Arduino/Genuino Mega or Mega 2560")
 }
+
+func TestBoardOptionsFunctions(t *testing.T) {
+	pm := packagemanager.NewPackageManager(
+		paths.New("testdata"),
+		paths.New("testdata"),
+		paths.New("testdata"),
+		paths.New("testdata"))
+	pm.LoadHardwareFromDirectory(paths.New("testdata"))
+
+	nano, err := pm.FindBoardWithFQBN("arduino:avr:nano")
+	require.Nil(t, err)
+	require.NotNil(t, nano)
+	require.Equal(t, nano.Name(), "Arduino Nano")
+
+	nanoOptions := nano.GetConfigOptions()
+	require.Equal(t, "Processor", nanoOptions.Get("cpu"))
+	require.Equal(t, 1, nanoOptions.Size())
+	nanoCPUValues := nano.GetConfigOptionValues("cpu")
+
+	expectedNanoCPUValues := properties.NewMap()
+	expectedNanoCPUValues.Set("atmega328", "ATmega328P")
+	expectedNanoCPUValues.Set("atmega328old", "ATmega328P (Old Bootloader)")
+	expectedNanoCPUValues.Set("atmega168", "ATmega168")
+	require.EqualValues(t, expectedNanoCPUValues, nanoCPUValues)
+
+	esp8266, err := pm.FindBoardWithFQBN("esp8266:esp8266:generic")
+	require.Nil(t, err)
+	require.NotNil(t, esp8266)
+	require.Equal(t, esp8266.Name(), "Generic ESP8266 Module")
+
+	esp8266Options := esp8266.GetConfigOptions()
+	require.Equal(t, 13, esp8266Options.Size())
+	require.Equal(t, "Builtin Led", esp8266Options.Get("led"))
+	require.Equal(t, "Upload Speed", esp8266Options.Get("UploadSpeed"))
+
+	esp8266UploadSpeedValues := esp8266.GetConfigOptionValues("UploadSpeed")
+	for k, v := range esp8266UploadSpeedValues.AsMap() {
+		// Some option values are missing for a particular OS: check that only the available options are listed
+		require.Equal(t, k, v)
+	}
+}