move all output modules to subdirectory, trash standard.nix

standard.nix isn't, is the essence here. Not all devices
support flashimage as it is currently defined - some
have diskimage, some have neither

vanilla-configuration-hw.nix

@@ -2,6 +2,10 @@
 {
   imports = [
     ./vanilla-configuration.nix
-    ./modules/standard.nix
+    ./modules/outputs/tftpboot.nix
+    ./modules/outputs/kexecboot.nix
+    ./modules/outputs/flashimage.nix
+    ./modules/outputs/jffs2.nix
+    ./modules/outputs/ubifs.nix
   ];
 }