Support prebuilt bootimage

Bug: 63676296
Test: `m -j` and inspect artifacts
Merged-In: Id32d8a7151a8e99b45aa58ee9971def9734eaf78
Change-Id: Id32d8a7151a8e99b45aa58ee9971def9734eaf78
(cherry picked from commit b0b1a24b1e039f9db25b6e549164af4992be188e)
1 file changed