Extensible SDK: allow for installation without preparing build system

Extensible SDK: allow for installation without preparing build system

When publishing SDK, what we want is basically its metadata and sstate
cache objects. We don't want the SDK to be prepared with running bitbake
as it takes time which reproduces meaningless output for the published SDK.

So this patch adds an option to allow for SDK to be extracted without
preparing the build system.

Signed-off-by: Chen Qi <Qi.Chen@windriver.com>