pkg/kconfig: support transitional line

[tarasmadan]

First used in:

config CFI_CLANG
        bool
        transitional
        help
          Transitional config for CFI_CLANG to CFI migration.

https://lwn.net/Articles/1039256/
I needed it for linux-next experiments. For the current upstream we don't need it.

P.S.
https://www.phoronix.com/news/Linux-6.18-Transitional explains the keyword usage.

[a-nogikh]

Please add it to https://github.com/google/syzkaller/blob/master/pkg/kconfig/kconfig_test.go

Overall, in pkg/kconfig we do try to replicate the Kconfig logic. So, ideally, we should not just parse, but also process the flag - that is, it should not be in the output of

syzkaller/pkg/kconfig/kconfig.go

Line 130 in 65a0eec

func ParseData(target *targets.Target, data []byte, file string) (*KConfig, error) {

If we ignore it, it will likely make config bisection (that we do at the beginning of a normal bisection) a tiny bit less efficient, so maybe in the end it's not worth it indeed. Still, would be nice to see a comment that we didn't actually process it intentionally.

[tarasmadan]

PTAL. Added the test and comment.

[tarasmadan]

Thank you @a-nogikh for the approval.
CCed others just FYI.

[melver]

FWIW, but probably no need to fix given this is already committed.