rename flashable->flashimage, o.squashfs to rootfs

"rootfs" describes what it is for, "squashfs" merely says
how it's implemented

(also, rootfs-as-jffs2 will soon be added)

tests/smoke/test.nix

@@ -5,7 +5,7 @@
 let img = (import liminix {
       device = import "${liminix}/devices/qemu/";
       liminix-config = "${liminix}/vanilla-configuration.nix";
-    }).outputs.squashfs;
+    }).outputs.rootfs;
     pkgs = import <nixpkgs> {};
 in pkgs.runCommand "check" {
   nativeBuildInputs = with pkgs; [ squashfsTools s6-rc ] ;