@Andrej, @Mickael:
that's a case of "ungraceful recovery when a site listed in a composite update site doesn't exist. It's my fault for an incomplete bootstrapping of the staging.previous/ folder. Because I'm lazy, I'm respinning http://hudson.qa.jboss.com/hudson/job/jbosstools-drools-5.3_trunk/ to produce a second build which will be identical to the one in staging/ folder, and the p2 site resolution warning should go away.