spec_file: handle update_macros needing a init'ed spec.
    
    When initing spec-file, update_macros() is invoked to let mock define
    various macros, either from mock buildroot or from prebuilt
    packages. In the latter case, spec needs to have a valid nvr.
    Walk around problem by parsing spec first once, setting release
    to '*' and then make the final parsing with proper release.