Move zlib, xz, and zstd to bcond
    
    This allows experiments to be run with reducing the size of the
    initramfs by having fewer bulky shared libraries in it, and also makes
    it easier for downstream distributions to make different configuration
    choices.
    
    We keep the defaults the same, building *with* all of these in order to
    not change what is practically being built in Fedora.
    
    Signed-off-by: Stewart Smith <trawets@amazon.com>