[IPv4] Add a config option for IPv4

IPv4 may not always be dragged into the link, so add a specific
configuration option for it.

src/config.h

@@ -74,6 +74,16 @@
 
 /* @END isa.h */
 
+/* @BEGIN general.h
+ *
+ * Network protocols
+ *
+ */
+
+#define	NET_PROTO_IPV4		/* IPv4 protocol */
+
+/* @END general.h */
+
 /* @BEGIN general.h
  *
  * Download protocols