net: Refactor to separate the UDP handler from the ARP handler

Call a built-in dummy if none is registered... don't require
protocols to register a handler (eliminating dummies)
NetConsole now uses the ARP handler when waiting on arp
(instead of needing a #define hack in arp.c)
Clear handlers at the end of net loop

Signed-off-by: Joe Hershberger <joe.hershberger@ni.com>
Acked-by: Simon Glass <sjg@chromium.org>
diff --git a/net/nfs.c b/net/nfs.c
index db73e93..5b99763 100644
--- a/net/nfs.c
+++ b/net/nfs.c
@@ -735,7 +735,7 @@
 		"Loading: *\b", load_addr);
 
 	NetSetTimeout(NFS_TIMEOUT, NfsTimeout);
-	NetSetHandler(NfsHandler);
+	net_set_udp_handler(NfsHandler);
 
 	NfsTimeoutCount = 0;
 	NfsState = STATE_PRCLOOKUP_PROG_MOUNT_REQ;