Support W5200 and W5500 Ethernet controllers

[per1234]

Merge support code from https://github.com/arjenhiemstra/SEM-Bootloader.

[SanSanysch]

I support! I have atmega2560 & w5500, but... not work! Is it worth waiting?

[per1234]

Hi @SanSanysch. Thanks for your interest in this project. Unfortunately, I have not done a good job of maintaining this.

It was suggested to me that I create a fork at the time I was actively working on this because the original repository codebndr/Ariadne-Bootloader was in a phase where it did not have a maintainer. But since then that project has revived and had some very important advancements, which I have not pulled into my fork.

So the best version to use is the one maintained by the original developer here:
https://github.com/loathingKernel/ariadne-bootloader

It has support for the W5500 already!

It does not have the Boards Manager support I added here, but doing a manual installation is easy enough.