build: Automatically set QCOM audio, display, media variants

* Device platform should determing the HAL that ought to be used. This
  commit forces QCOM_HARDWARE to select the -caf HAL variants, which
  are then broken down by platform within the variant path.

Change-Id: I6fc7a3def7b93112f034a3b89552f302727cdbf8

Conflicts:
	core/qcom_target.mk