sdk: Fix keeping unset as unset

The configuration preservation mechanism fails to keep items was are unset
as unset.  For busybox this causes issues when building with custom
settings in the SDK.  Therefore preserve busybox unset settings.
In addition we preserve non-package selection unset settings because
they may be important for making sure we compile and identical
package for packages that are recompiled in the SDK.

Now that SDK not longer ships .config this becomes relevant for
any use of the SDK.

Signed-off-by: Daniel Dickinson <lede@daniel.thecshore.com>