cmake: Support custom out-of-tree toolchains

Add a new optional TOOLCHAIN_ROOT cmake and environment variable to
specify an alternative location for toolchain cmake files.

When set, Zephyr will look for a toolchain cmake file located in:
${TOOLCHAIN_ROOT}/cmake/toolchain/${ZEPHYR_TOOLCHAIN_VARIANT}.cmake

Signed-off-by: François Delawarde <fnde@oticon.com>