Extend the usage of TARGET_KERNEL_CUSTOM_TOOLCHAIN
TARGET_KERNEL_CUSTOM_TOOLCHAIN := 4.6 or 4.7 or 4.8-linaro or 4.9-linaro
        If any other selected, or none selected it will
        use 4.7

Change-Id: Id671ba2c03b4d420818deadfd4f0ee83408be724