add cm custom boot image processing.
This varies from the AOSP method
CM use BOARD_CUSTOM_BOOTIMG_MK which is a path to a makefile fragment
This file contains a custom boot image and recovery image creation target
which replace the defaults found in build/core/Makefile

AOSP uses BOARD_CUSTOM_MKBOOTIMG which is the name of the binary to use
instead of mkbootimg.
Updated Kernel uses philz recovery
https://github.com/PhilZ-cwm6/philz_touch_cwm6/tree/cm-11.0

Cold Boots in Android
Splash Screen Recovery Switch
Adjust include paths to enable building under cm and aosp source
trees
Update BoardConfig and add BoardConfigProduct.mk for clean filtering