vboot: Add validation checks for AVB verification data

In vb2_load_android, the result of avb_slot_verify is used to determine
verification success. To ensure the returned verification data matches
the expected boot state, introduce verify_avb_data() to perform additional
checks.

BRANCH=none
BUG=b:518008064
TEST=Build and boot test

Change-Id: I93842945dda245da8176bbc805e9ad7556bcd7d4
Signed-off-by: Grzegorz Bernacki <bernacki@google.com>
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/vboot_reference/+/8063940
Reviewed-by: Jakub "Kuba" Czapiga <czapiga@google.com>
Reviewed-by: Julius Werner <jwerner@chromium.org>