add per-device overlay

presently this is used to reference the appropriate kernel and (if
needed) openwrt source trees, but I would not be surprised if we find
other uses

tests/module/run.sh

@@ -1,2 +1,2 @@
 set -e
-NIXPKGS_ALLOW_UNSUPPORTED_SYSTEM=1 nix-build --arg device "import <liminix/devices/qemu.nix>" test.nix
+NIXPKGS_ALLOW_UNSUPPORTED_SYSTEM=1 nix-build --arg device "import <liminix/devices/qemu>" test.nix