cmake: Introduce optional Kconfig for toolchains

Allow a given toolchain to specify Kconfig options that might be
relevant to a feature available in that toolchain.

For example, the ARM embedded GNU toolchain supports two variants of
newlib and you select the smaller one via a spec file.  We can use a
Kconfig option like HAS_NEWLIB_LIBC_NANO to convey that this feature is
supported by that toolchain.

We look for the toolchain Kconfig in ${TOOLCHAIN_KCONFIG_DIR}/Kconfig,
and default TOOLCHAIN_KCONFIG_DIR to:

${TOOLCHAIN_ROOT}/cmake/toolchain/${ZEPHYR_TOOLCHAIN_VARIANT})

toolchain specific cmake files can override the default if needed.

Additionally tweaked the zephyr/generic.cmake to use
${CMAKE_CURRENT_LIST_DIR} to reduce some duplication.

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>
Signed-off-by: Sebastian Bøe <sebastian.boe@nordicsemi.no>
Signed-off-by: Kumar Gala <kumar.gala@linaro.org>