commands: add the 'sdk' command to manage SDK.

This command can list and install SDK.

Run 'west sdk install' to install the SDK.
Run without any parameter, installing
specified by SDK_VERSION in the source tree.
'west sdk' to show installed SDK information.

This command is just a wrapper for SDK's setup command,
but it simplifies the installation process. It will be a good
improvement for onboarding first-time users.

Signed-off-by: TOKITA Hiroshi <tokita.hiroshi@gmail.com>