Update notes for submitted changes

* 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