core: move platform-specific helper macros to target makefiles

* These (qcom specifically) are now needed sooner than we had
  previously been importing them. Include them within their
  <platform>_target.mk makefiles. This ensures they'll be
  imported before their macros are needed.

Change-Id: I06c6ab66446e2f0b54c245cf6c2cf665b649e0c9