cmake: get: using NOT DEFINE BUILD_VERSION instead of NOT BUILD_VERSION

If user is specifying `-DBUILD_VERSION=<value>` then the proper way to
test is using `NOT DEFINED BUILD_VERSION`.

The difference between `NOT DEFINED BUILD_VERSION` and
`NOT BUILD_VERSION` is that a user specifying `-DBUILD_VERSION=0` will
result in `NOT BUILD_VERSION` becoming true, whereas
`NOT DEFINED BUILD_VERSION` would be false, which is the correct
behavior here, as the user actually provided a specific BUILD_VERSION.

Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>