cm: CM-specific build macros

 * vendor/*/build/core/definitions.mk is automatically sourced by the
   build system to load custom macros. Start using this for some
   of our own.
 * Adds a "uniq" macro to remove duplicates from a list without changing
   it's order.

Change-Id: Id5f1eb4b9f81bb31ca0b3d5e74c298b3d105da10