Refactor and optimize Ethernet library

[rowingdude]

Using const liberally so that the compiler can try to optimize this down a bit more.

I put in a couple nullptr checks to help stability as I've had my controller crash, hopefully this helps someone.

I created a configure_static_ip function so that int CEthernet::begin ... can remain function specific.

Set ni to a nullptr to avoid any potential "use after free" issues.

[CLAassistant]

All committers have signed the CLA.

[JAndrassy]

could you please move configureStaticIP bellow the begin which had that code so it is compared in the diff

[rowingdude]

Yes, sorry. I got side-tracked adding those annoying /------ ... --/s

[rowingdude]

I will go hunt through the libraries/Wifi/Examples and update as needed to get things moving. I feel like I'm going to end up porting all of the newer WiFi code over to Arduino.

[JAndrassy]

you forgot to commit Ethernet.h?

and please try to eliminate unnecessary differences
https://github.com/arduino/ArduinoCore-renesas/pull/357/files

[rowingdude]

I will do that, there is no "Ethernet.h" in the original directory, so I did not think to include it.
However, thank you for your feedback, these big projects are new to me and I'm learning as I go.

[rowingdude]

I'm going to close this until I work out the bugs. Thank you, initially my local repo didn't have Ethernet cloned into it and I was going off the web version, however, with the new update, I cannot submit this as is. Cheers