Add virtual package for FDE unlocker dependency

This adds a 'virtual package' called postmarketos-fde-unlocker, and sets osk-sdl and a new postmarketos-base-nofde pacakges to provide it. It's in preparation for, and required by !2242 (merged).

This is split off from !2242 (merged), to build the virtual package that postmarketos-mkinitfs depends on first so that CI won't fail.