Use LKROOT as the path to LK am: ac0c10dc51

Original change: https://android-review.googlesource.com/c/trusty/app/avb/+/2870900

Change-Id: Ic0856cbfaa855ff1aaccf36e4a015064fac1c7ff
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>