scripts: Handle relative build-root paths am: 4f292492b5

Original change: https://android-review.googlesource.com/c/trusty/vendor/google/aosp/+/2980838

Change-Id: I3f67bae4907cc5470f2d329f900a40c2256ce9e8
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>