That way we avoid recompiling source files that don't depend on vehicles.
Change notes:
- UTILITY_SOURCE_EXTS: moved to ap_library
- Task generators indexes: ap_library handles that now and in a better way
- Use of AP_STLIB_FEATURES: it doesn't make sense anymore, since the stlib
taskgen has empty source list
- Flags and defines: passed down to ap_library through AP_LIBRARIES_OBJECTS_KW
- Set use='mavlink' in AP_LIBRARIES_OBJECTS_KW instead of calls to
bld.ap_stlib().
b8e518a533