cmake: kconfig: Expose ZEPHYR_TOOLCHAIN_VARIANT to Kconfig

Expose ZEPHYR_TOOLCHAIN_VARIANT so that Kconfig can make choices based
on how its set.  For example we RISCV_GENERIC_TOOLCHAIN if the variant
is 'zephyr'.

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>