Only include uncrypt if not in system/core

Some android trees have uncrypt in system/core (cm-11.0). This breaks
building recovery since two modules of the same name conflict. Only
include the local copy of uncrypt if it doesn't already exist in
system/core.

Change-Id: Ie2d7cdedf0d4c9ad148c43b317d1f22d1e50e1f7