am 1b6d0a65: allow prebuilts to specify LOCAL_CERTIFICATE so they can be resigned

Merge commit '1b6d0a65fc62901f2c18d00c556bd5da50f35584' into eclair

* commit '1b6d0a65fc62901f2c18d00c556bd5da50f35584':
  allow prebuilts to specify LOCAL_CERTIFICATE so they can be resigned