Empty-initialize verified_boot_key when unverified

Per https://source.android.com/security/keystore/attestation the
verified_boot_key needs to be empty if we are in the unverified state.
This change makes Trusty robust to bootloaders providing it a
verified_boot_key that was not used to verify the image.

Bug: 138606521
Test: atest CtsKeystoreTestCases; atest VtsHalKeymasterV4_0TargetTest
Change-Id: I05e94243405c680281d562e5185299c6da556e0b
1 file changed