Build - 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>

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: I4e9de66ea721c6e662955664ed17bf39be14a7c3
Signed-off-by: STELIX <ssspinni@gmail.com>