Revert "otasigcheck: Go back to the key-rewrite check."

sigcheck now returns an explicit abort code

This reverts commit bab150d79c7b9b1f233fc3592b69a42f0b38fbd8.

Change-Id: I2b5860ea427a4db7e29b55cc632b92c6e2910494
Merge remote-tracking branch 'cyanogen/cm-12.0' into HEAD