cmake: Zephyr sdk package handling

This commit introduces Zephyr SDK CMake config package.

This removes the need for setting ZEPHYR_SDK_INSTALL_DIR and
ZEPHYR_TOOLCHAIN_VARIANT when using Zephyr SDK in Linux.
It also allows to introduces never SDKs without breaking Zephyr.

For example, with this PR, the current Zephyr SDK is 0.11.1 but when
releasing 0.12 then the current Zephyr will no longer built.
This PR moves the Zephyr SDK CMake related code to the SDK and thus
allowing to use newer SDKs, as long as they are backwards compatible.

It also allows multiple SDK installations, and will automatically select
the version closet to the required version.

Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>