this attempts to resolve

https://github.com/kubernetes/minikube/issues/2768
for Windows, by converting \ in directory string to / and using
`path.Join` instead of `filepath.Join` which uses Unix format
instead of using he current OS directory delimeter.

pkg/minikube/assets/addons.go

@@ -20,6 +20,7 @@ import (
 	"fmt"
 	"os"
 	"path/filepath"
+	"path"
 	"strconv"
 
 	"github.com/pkg/errors"
@@ -305,7 +306,8 @@ func addMinikubeDirToAssets(basedir, vmpath string, assets *[]CopyableFile) erro
 					return errors.Wrap(err, "generating relative path")
 				}
 				rPath = filepath.Dir(rPath)
-				vmpath = filepath.Join("/", rPath)
+				rPath = filepath.ToSlash(rPath)
+				vmpath = path.Join("/", rPath)
 			}
 			permString := fmt.Sprintf("%o", info.Mode().Perm())
 			// The conversion will strip the leading 0 if present, so add it back