policy: don't link endpoints on startup

Sometimes the default device node might not exist when reevaluating endpoints
for the first time on startup, so the policy would link endpoints to another
device node. Then, the default device node appears and the policy moves the
endpoints to the default device node while the previous link has not finish its
activation yet. This race condition can cause endpoint links to fail when being
activated. Delaying the reevaluation of endpoint links until the first client
link is created avoids this issue.