Add XB-Efi-Vendor metadata to efi-*-bin packages

Downstream distributions that do not rebuild grub but want to support Secure Boot need to know the path used when building the monolothic EFI images, as that's necessary to create the layout and the grub.cfg files in the live media. Export the path, which is the distribution vendor, in a control file metadata variable so that it can be programmatically accessed.