add "standard" module, which includes flashimage kexec & jffs2

most systems need most of these, so it makes writing the docs a
lot easier

modules/standard.nix

@@ -0,0 +1,11 @@
+{
+  # "standard" modules that aren't fundamentally required,
+  # but are probably useful in most common workflows and
+  # you should have to opt out of instead of into
+  imports = [
+    ./tftpboot.nix
+    ./kexecboot.nix
+    ./flashimage.nix
+    ./jffs2.nix
+  ];
+}