A couple of 6.3.0 fixes

[avtolstoy]

Description

1. Wiznet W5500 usual start up time is up to 250ms, so need to retry for longer. (this was broken in [network] postpone ethernet interface initialization to allow STARTUP() call to override pin configuration without a reset #2847 and was not in any public release)
2. EEPROM.get/set cannot be used with String objects, generate assertion failure when doing so
3. Update network tests to stress test large packets up to MTU size once max MTU is figured out

References

Links to the Community, Docs, Other Issues, etc..

---

Completeness

• User is totes amazing for contributing!
• Contributor has signed CLA (Info here)
• Problem and Solution clearly stated
• Run unit/integration/application tests on device
• Added documentation
• Added to CHANGELOG.md after merging (add links to docs and issues)