build-aux: Use dpkg-buildpackage options to not attempt signing anything
Currently riscv64 builds are failing, because dpkg-buildpackage
attempts signing. Use the historic `-uc -us` (unsigned changes,
unsigned sources) options that are available in xenial and up, to
prevent attempts to sign with mvo's gpg key inside launchpad snap
builds.
cmd/snap,image: fix unit tests in cmd_prepare_image, add unit test case validation sets where revisions are not specified, and where presence is invalid
image: redo the seed manifest functionality, don't use interfaces and rely on the type system. Better support for validation sets, and don't output them when controlled by a validation set