releasetools: Android T GKI certification scheme

Companion change of Iaf48a6e3d4b97fa6bfb5e1635a288b045baa248f
To support new GKI certification scheme for boot.img and
init_boot.img on upgrading and launching device combinations.

Bug: 210367929
Bug: 211741246
Bug: 203698939
Test: atest --host releasetools_test:test_common
Test: unpack_bootimg --boot_img boot.img
Test: unpack_bootimg --boot_img init_boot.img
Test: avbtool info_image --image out/boot_signature
Change-Id: I3749297c09c3899046550e4be776acbeea37ef2e