releasetools: Update T GKI certification scheme

Companion change of I143680b1cab50a6915df56c8273f8741beaf1180.
Basically does the same thing as the other change.

Bug: 211741246
Test: m dist
Test: ./boot_signature_info.sh boot-5.10.img
Change-Id: I40c4d5866c74a9a2d525f9455969b8a71f22bdbb