Gummiboot is now part of the core, but the infrastructure supporting efi is all grub-efi specific. mkefidisk needs to be more generic to handle an efi-plugin and other abstractions.
I have updated mkefidisk to support gummiboot. THere isn't a plugin architecture per-se - but I think that can be left to wic, and leave mkefidisk.sh as a simple single file installer that can be easily distributed with images. Gummiboot specifically added in: 88045c9 mkefidisk.sh: Add gummiboot support
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