exynos_omx: Add BOARD_USE_KHRONOS_OMX_HEADER flag.

exynos_omx: Add BOARD_USE_KHRONOS_OMX_HEADER flag.

BOARD_USE_KHRONOS_OMX_HEADER flag is needed to include proper openMAX
spec header file.

If BOARD_USE_KHRONOS_OMX_HEADER is true, use khronos's original omx
header files.
    - $(EXYNOS_OMX_TOP)/include/khronos/
else, use android's omx header files.
    - frameworks/native/include/media/openmax

Google extends OMX header files of Khronos.
    - OMX_COLOR_FormatAndroidOpaque was added in OMX_COLOR_FORMATTYPE.
    - OMX_VIDEO_CodingVPX was added in OMX_VIDEO_CODINGTYPE.
    - FLAC type for audio was added.
    - etc..

So this BOARD_USE_KHRONOS_OMX_HEADER flag is needed to avoid conflict
error about OMX_COLOR_FormatAndroidOpaque, etc.

Change-Id: Ib03ad0ea83669e58125da32d590368ac11d36677
Signed-off-by: Yunji Kim <yunji.kim@samsung.com>
Signed-off-by: Huisung Kang <hs1218.kang@samsung.com>