Don't build default initrd for directory images by default

Don't build default initrd for directory images by default

We switched this around a long time ago to accomodate virtiofsd qemu
boots as those needed an initrd without the image necessarily being
bootable. Given Fedora and Arch now allow for direct kernel boot with
virtiofsd without needing an initrd, let's switch back the default to
not building an initrd for directory images, so that the same config
can be used to build disk images for vm boot and directory images for
container boot without having to build the initrd when building directory
images.