cmake: Support SDK versions more broadly

The SDK version is of the form X.Y.Z.  Change the cmake scripts to be
based on X.Y of the version.  This allows us to easily support newer
toolchains without having to explicitly add cmake files for the version
as well as removes duplication between those files.

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>
Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>