MCUdude's cores no longer use optiboot

[vanyasem]

As of 3.0.0 MCUdude's cores are now using urboot as a bootloader instead of Optiboot.

I believe it's worth mentioning that in the README.

[WestfW]

In particular, the avrdude support for urboot, which is supposed to fall back to Optiboot-compatible, will not do so if you are running a customized optiboot.
avrdudes/avrdude#1120