gs101: re-enable prebuilt boot.img/boot-user.img usage am: 7badc4b5f1

Original change: https://googleplex-android-review.googlesource.com/c/device/google/gs101/+/15934983

Change-Id: Ib6489b4b5797eea2b8ecc32ee10e3c4d1998f4d8