UEFI: add option to set override EFI/vendor directory

When using Secure Boot, grub2 as built by Debian will now load a config file from EFI/$VENDOR instead of having EFI/debian hardcoded. $VENDOR comes from dpkg-vendor or from the user building grub2. Use dpkg-vendor from the chroot by default, and let users override it via a new --uefi-vendor option.

The changes to Grub are here: https://salsa.debian.org/pmhahn/grub/commits/signing and the process to merge them back into sid is starting, so we should get ready.