Fuse GRUB configuration into the EFI binary
Value
Updating two files in a safe, transactional manner is way, way harder than updating one. By having the grub.cfg file built-into GRUB binary we can avoid a lot of complexity without limiting anything relevant.
Description
Configure GRUB to have a built-in config file identical to the one we currently build in the oniro-grub-bootconf recipe. The existing file may "stay" if it is unused and this makes things easier for wic integration. Make sure to create a cleanup task if this happens.