keymint: add in sha-256 impl am: 8b2f8486b8

Original change: https://android-review.googlesource.com/c/trusty/app/keymint/+/2787080

Change-Id: I28da9e4ae6d1e272cf1bd16f3af2f9503a74988f
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>