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/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
 ''