Fix key setup when preseeding a local repository
When preseeding a local repository via apt-setup/localX/repository, the repository key for Secure Apt needs to be configured with apt-setup/localX/key. This key used to be setup with apt-key, but it's use is deprecated and apt's former dependency on gnupg has been demoted to a Suggests, rendering apt-key non-functional in d-i. Apply a patch by Lars Kollstedt (thanks!) which adds the repository key(s) to /etc/apt/trusted.gpg.d, following the approach used by pbuilder (Closes: #851774)