cmake: set system name correctly

For unknown reasons, the cmake class is using SDK_OS as the
target system OS. This makes no sense but only shows up as a problem
when you try a different SDK OS. Fix it to use TARGET_OS which is
the correct thing to do. For the vast majority of users this will
make no difference.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>