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/pppoe/test.nix

@@ -46,7 +46,7 @@ trap fatal ERR
 
 routeros $serverstatedir
 mkdir vm
-mips-vm --background ./vm ${img}/vmlinux ${img}/squashfs
+mips-vm --background ./vm ${img}/vmlinux ${img}/rootfs
 expect ${./getaddress.expect}
 
 set -o pipefail

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 ] ;

tests/wlan/test.nix

@@ -34,6 +34,6 @@ fatal(){
 trap fatal ERR
 
 mkdir vm
-mips-vm --background ./vm ${img}/vmlinux ${img}/squashfs
+mips-vm --background ./vm ${img}/vmlinux ${img}/rootfs
 expect ${./wait-for-wlan.expect} |tee output && mv output $out
 ''