introduce modules/arch/{mipsel,mipseb}.nix

for settings that are common to all mipse[lb] but would not be shared
with e.g. aarch64 or x86

modules/arch/mipsel.nix

@@ -0,0 +1,9 @@
+{ lib, pkgs, config, ...}:
+{
+  imports = [ ./mips.nix ];
+  config = {
+    kernel.config = {
+      CPU_LITTLE_ENDIAN = "y";
+    };
+  };
+}