This makes it possible to avoid building components which are not used
in the native or runtime tooling. Most users of nanopb just want to
build `nanopb-generator-native` and `nanopb-runtime` and not the other
two combinations as they are building software for their target
leveraging nanopb.
Signed-off-by: William A. Kennington III <wak@google.com>
Signed-off-by: Khem Raj <raj.khem@gmail.com>