generate a mips squashfs with some s6-rc services

haven't even tried booting it but I don't expect it to work. Needs

- s6-linux-init or some other pid 1 program
- a kernel

devices/gl-ar750.nix

@@ -0,0 +1,11 @@
+{
+  system = {
+    crossSystem = {
+      config = "mips-unknown-linux-musl";
+      gcc = {
+        abi = "32";
+        arch = "mips32";          # maybe mips_24kc-
+      };
+    };
+  };
+}