pkg/kconfig: support transitional line

See https://www.phoronix.com/news/Linux-6.18-Transitional .

First used in:
config CFI_CLANG
        bool
        transitional
        help
          Transitional config for CFI_CLANG to CFI migration.

Author: tarasmadan

pkg/kconfig/kconfig.go

@@ -337,6 +337,10 @@ func (kp *kconfigParser) parseProperty(prop string) {
 		kp.ConsumeLine()
 	case "modules":
 	case "optional":
+	// transitional is used for the configs backward compatibility.
+	// We can ignore them. After the config final removal we'll see the kconf error.
+	// https://www.phoronix.com/news/Linux-6.18-Transitional
+	case "transitional":
 	case "default":
 		kp.parseDefaultValue()
 	case "range":

pkg/kconfig/kconfig_test.go

@@ -21,6 +21,13 @@ func TestParseKConfig(t *testing.T) {
 mainmenu "test"
 config FOO
 	default "$(shell,$(srctree)/scripts/gcc-plugin.sh "$(preferred-plugin-hostcc)" "$(HOSTCXX)" "$(CC)")" if CC_IS_GCC
+`,
+		},
+		{
+			in: `
+mainmenu "test_transitional"
+config FOO
+	transitional
 `,
 		},
 	}