allow to specify a full path for a custom kernel toolchain

example:

TARGET_KERNEL_CUSTOM_TOOLCHAIN_FULL := \
        prebuilts/gcc/linux-x86/arm/arm-cortex_a8-linux-gnueabi-linaro_4.9.1-2014.04/bin/arm-gnueabi-

Change-Id: I076480f9752bb90cbc7594435c8410fcc1668490