build: qcom: Set QCOM_*_PATH variables for hardware/qcom-* projects

This consolidates a bunch of one-off logic scattered throughout the
code base.  Usage in Android.mk files is trivial:

At top level, use e.g. "ifeq ($(call my-dir),$(QCOM_AUDIO_PATH))".
This works for all variants, including non-variants (i.e. AOSP).

Within subdirs, use e.g. hardware/qcom/audio => $(QCOM_AUDIO_PATH)

Change-Id: Iee2497ea9a7efeb4ae9e861b84c532b19da7b69d
build: Introduce project pathmap and use it for qcom variants

The project pathmap tracks the path to the top level of a project.  The
following functions are provided:

 * project-set-path adds an entry.
 * project-set-path-variant adds a "variant" entry.
 * project-path-for retrieves an entry.

To use as a guard in Android.mk:

  ifeq ($(call my-dir),$(call project-path-for,projectname))

To use for include paths in Android.mk:

  LOCAL_C_INCLUDES += $(call project-path-for,projectname)/...

Set project pathmap for qcom project variants.

Change-Id: I8dceca72a1ba80fc7b1830c5ab285d444f530457
build: Add "dtbs" target when building the kernel

 * This is needed for 3.10

Change-Id: I4044ea9e67017452efc25097a3327141a6627c24
build: Set QCOM variants for non-QCOM_HARDWARE defined targets

* Nexus devices and others typically do not define the QCOM_HARDWARE
  flag, so the variant path should always default to the AOSP variant.
* Unconditionally set the variant to the AOSP HAL by default.

Change-Id: I714170897128f92718af266366cfcbf3136e8981