Add define vendor to prjconf

In order to allow use of the vendor macro in prjconf, add a define for
it in addition of the `Macro:` definition.

As this will get defined just after the `_project` one this shouldn't
have any side-effect even if some users are currently defining it in
their own configuration.

Signed-off-by: Nicolas Belouin <[email protected]>

src/backend/bs_srcserver

@@ -2558,6 +2558,7 @@ sub concatconfigs {
   }
   $vprojid = $projid if $vprojid eq '';
   my $obsname = $BSConfig::obsname || 'build.opensuse.org';
+  $config .= "%define vendor obs://$obsname/$vprojid\n";
   $macros .= "%vendor obs://$obsname/$vprojid\n";
   $macros .= "%_download_url $BSConfig::repodownload\n" if $BSConfig::repodownload;