Fix cpb traverses unneeded paths

cpb, it is run on '/' of several images, outputs many unuseful messages
and spoils log aggregation systems.  So cpb excludes device filesystems
and pseudo filesystems from filesystems looking for metadata.

Signed-off-by: ERAMOTO Masaya <eramoto.masaya@jp.fujitsu.com>

util/cpb/main.go

@@ -84,14 +84,32 @@ func getMetadata() (m *metadata, err error) {
 		manifestDir:     "/manifests",
 	}
 
+	// Exclude device filesystems and pseudo filesystems from filesystems looking for metadata
+	excludeDir := []string{
+		"/dev",
+		"/proc",
+		"/sys",
+	}
+
 	// Traverse the filesystem looking for metadata
 	err = filepath.Walk(".", func(path string, info os.FileInfo, err error) error {
 		if err != nil {
 			fmt.Printf("prevent panic by handling failure accessing a path %q: %v\n", path, err)
 			return nil
 		}
 		if info.IsDir() {
-			fmt.Printf("skipping a dir without errors: %+v \n", info.Name())
+			absPath, err := filepath.Abs(path)
+			if err != nil {
+				fmt.Printf("couldn't get the absolute path %q: %v\n", path, err)
+				return nil
+			}
+			for _, v := range excludeDir {
+				if v == absPath {
+					fmt.Printf("skipping all files in the dir: %+v \n", absPath)
+					return filepath.SkipDir
+				}
+			}
+			fmt.Printf("skipping a dir without errors: %+v \n", absPath)
 			return nil
 		}
 		if info.Name() != bundle.AnnotationsFile {