trusty:keybox: only build the fake keybox if configured am: 3a2e86e6df

Original change: https://android-review.googlesource.com/c/trusty/app/sample/+/2950426

Change-Id: Ica9ffdc458c1fb8358a6bb115a6f5f8b66b53cbf
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>