Support non-binary kconfig options in kernel feature source

Discover other than bool or tristate kconfig options, too. For bool and
tristate the node label is still binary (i.e. set to "true" if the
kconfig option has been enabled). For other kconfig types (e.g. string
or int) the value of the label directly corresponds to the value of the
kconfig flag, e.g. "32", "elf64-x86-64" etc.

source/kernel/kernel.go

@@ -25,6 +25,7 @@ import (
 	"regexp"
 	"strings"
 
+	"k8s.io/apimachinery/pkg/util/validation"
 	"sigs.k8s.io/node-feature-discovery/source"
 )
 
@@ -72,8 +73,8 @@ func (s Source) Discover() (source.Features, error) {
 
 	// Check flags
 	for _, opt := range Config.ConfigOpts {
-		if _, ok := kconfig[opt]; ok {
-			features["config."+opt] = true
+		if val, ok := kconfig[opt]; ok {
+			features["config."+opt] = val
 		}
 	}
 
@@ -133,8 +134,8 @@ func readKconfigGzip(filename string) ([]byte, error) {
 }
 
 // Read kconfig into a map
-func parseKconfig() (map[string]bool, error) {
-	kconfig := map[string]bool{}
+func parseKconfig() (map[string]string, error) {
+	kconfig := map[string]string{}
 	raw := []byte(nil)
 	err := error(nil)
 
@@ -177,7 +178,14 @@ func parseKconfig() (map[string]bool, error) {
 	for _, line := range lines {
 		if m := re.FindStringSubmatch(string(line)); m != nil {
 			if m[2] == "y" || m[2] == "m" {
-				kconfig[m[1]] = true
+				kconfig[m[1]] = "true"
+			} else {
+				value := strings.Trim(m[2], `"`)
+				if len(value) > validation.LabelValueMaxLength {
+					logger.Printf("WARNING: ignoring kconfig option '%s': value exceeds max length of %d characters", m[1], validation.LabelValueMaxLength)
+					continue
+				}
+				kconfig[m[1]] = value
 			}
 		}
 	}