It is a myth that /dev/urandom is insecure, /dev/urandom is cryptographically secure. The difference between /dev/urandom and /dev/random has no practical impact whatsoever on the security of the cryptographic protocols used within Elasticsearch (via X-Pack) (random numbers seeded by /dev/random or /dev/urandom are generated elsewhere within Elasticsearch (e.g., for generating UUIDs) but these have no intention of being secure).
Also, I'm not sure what this has to do with startup, there are not issues here like JRuby startup (that impacts Logstash).