expose TARGET_ARM_EABI_VERSION and TARGET_KERNEL_CUSTOM_TOOLCHAIN to dumpvar

Usage:
        TARGET_ARM_EABI_VERSION := 4.6 or 4.7 or 4.8
        For selecting another GCC version to be used.
        Default is 4.7 if not set, or wrong set.
        Note: We don't have 4.8 TC in our manifest yet.

        TARGET_KERNEL_CUSTOM_TOOLCHAIN := 4.6 or 4.7 or 4.8-linaro
        Those 3 exsist in our manifest.
	Any other TC can be used if they exsist in : prebuilts/gcc/linux-x86/arm/ and are named arm-linux-androideabi- and so on
	Default is 4.7

Change-Id: I63a35a15dcf9fc4c5915e70238c22c2978691951