WIP: Add plymouth and themes

Required to make splash screen appear on all architectures.

While the setup/configuration code for images is not yet present, the packages will be needed regardless of the outcome.

Because the configuration must happen in image-builder, which doesn't have a place to specify additional packages out of band, it is necessary to have dependencies online in the metapackage to do proper end-to-end testing of the splash screen.

WIP because flash-kernel and its initramfs are not there yet.