Allow a device to generically define its own headers

We have a few cases of devices including specific versions of projects
just because of modified headers (msm_mdp.h comes to mind), and I just
had enough of ifdeffing header files for specific cases (the P990 needs
a lot of these).
Now... if a target defines a TARGET_SPECIFIC_HEADER_PATH, any headers in
there will take precedence over the standard ones; for example, on the
p990, I have

TARGET_SPECIFIC_HEADER_PATH := device/lge/p990/include

which makes, for example, the
device/lge/p990/include/hardware_legacy/AudioHardwareInterface.h be
used instead of
hardware/libhardware_legacy/include/hardware_legacy/AudioHardwareInterface.h
whenever a source file uses <hardware_legacy/AudioHardwareInterface.h>

Change-Id: I41b62668b60e3f62a6ebd3738d8d2675103a81e6a

build: fix target header overlay

LOCAL_C_INCLUDES as defined by the makefile should be put AFTER
the overlay includes so the overlay always takes precedence.

Change-Id: I489b2aab6dbacd9122d834f85e07b63ed1271f07

core/binary.mk

@@ -1375,6 +1375,11 @@ my_tracked_gen_files :=
 $(foreach f,$(my_tracked_src_files),$(eval my_src_file_obj_$(s):=))
 my_tracked_src_files :=
 
+## Allow a device's own headers to take precedence over global ones
+ifneq ($(TARGET_SPECIFIC_HEADER_PATH),)
+my_c_includes := $(TOPDIR)$(TARGET_SPECIFIC_HEADER_PATH) $(my_c_includes)
+endif
+
 my_c_includes += $(TOPDIR)$(LOCAL_PATH) $(intermediates) $(generated_sources_dir)
 
 # The platform JNI header is for platform modules only.