Add support for selectable kernel toolchains set in device BoardConfig.mk

TARGET_KERNEL_CUSTOM_TOOLCHAIN= linaro
     builds kernel with linaro toolchain in: prebuilts/gcc/linux-x86/toolchain/linaro/bin/arm-eabi-

TARGET_KERNEL_CUSTOM_TOOLCHAIN= arm-eabi-4.6/bin/arm-eabi-
     builds kernel with arm-eabi-4.6 toolchain in: prebuilts/gcc/linux-x86/arm/arm-eabi-4.6/bin/arm-eabi-

TARGET_KERNEL_CUSTOM_TOOLCHAIN= arm-eabi-4.7/bin/arm-eabi-
     builds kernel with arm-eabi-4.7 toolchain in: prebuilts/gcc/linux-x86/arm/arm-eabi-4.7/bin/arm-eabi-