Kernel.mk: Add options to build with optimized cotex-a15, cortex-a9 or generic linaro toolchains
* Use one of the following options in boradconfig.mk as desired:
 TARGET_KERNEL_CUSTOM_TOOLCHAIN := 4.9-linaro or
 TARGET_KERNEL_CUSTOM_TOOLCHAIN := 4.8-linaro or
 TARGET_KERNEL_CUSTOM_TOOLCHAIN := 4.9-linaro-a9 or
 TARGET_KERNEL_CUSTOM_TOOLCHAIN := 4.8-linaro-a9 or
 TARGET_KERNEL_CUSTOM_TOOLCHAIN := 4.9-linaro-a15 or
 TARGET_KERNEL_CUSTOM_TOOLCHAIN := 4.8-linaro-a15

Change-Id: I661ba13919b5c102d7dd4546d11e6162890d5723