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

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

examples/arhcive.nix

@@ -31,10 +31,8 @@ in rec {
   };
 
   imports = [
-    ../modules/tftpboot.nix
+    ../modules/standard.nix
     ../modules/wlan.nix
-    ../modules/flashimage.nix
-    ../modules/kexecboot.nix
   ];
 
   hostname = "arhcive";

examples/extneder.nix

@@ -34,8 +34,7 @@ in rec {
 
   imports = [
     ../modules/wlan.nix
-    ../modules/tftpboot.nix
-    ../modules/flashimage.nix
+    ../modules/standard.nix
   ];
 
   hostname = "extneder";

examples/rotuer.nix

@@ -33,9 +33,7 @@ in rec {
 
   imports = [
     ../modules/wlan.nix
-    ../modules/tftpboot.nix
-    ../modules/flashimage.nix
-    ../modules/jffs2.nix
+    ../modules/standard.nix
   ];
   rootfsType = "jffs2";