net: add a generic udp protocol

This commit adds a generic udp protocol framework in the
network loop. So protocol based on udp may be implemented
without modifying the network loop (for example custom
wait magic packet).

Signed-off-by: Philippe Reynes <philippe.reynes@softathome.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/include/net.h b/include/net.h
index 1bf9867..2191071 100644
--- a/include/net.h
+++ b/include/net.h
@@ -551,7 +551,7 @@
 
 enum proto_t {
 	BOOTP, RARP, ARP, TFTPGET, DHCP, PING, DNS, NFS, CDP, NETCONS, SNTP,
-	TFTPSRV, TFTPPUT, LINKLOCAL, FASTBOOT, WOL
+	TFTPSRV, TFTPPUT, LINKLOCAL, FASTBOOT, WOL, UDP
 };
 
 extern char	net_boot_file_name[1024];/* Boot File name */