Stub KM_PURPOSE_ATTEST_KEY support.

Until Trusty's Keymaster implementation is upgraded to KeyMint, we
don't actually need to support ATTEST_KEY, but we do need to tweak
some things to accomodate relevant interface changes.

Bug: 171845787
Test: build.py qemu-generic-arm64-gicv3-test-debug --test=android-test:vts:keymaster@4.0
Change-Id: I883a1110d6649b2844bf2b74c38c5602d776774e
4 files changed