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>

net/dns.c

@@ -200,7 +200,7 @@ DnsStart(void)
 	debug("%s\n", __func__);
 
 	NetSetTimeout(DNS_TIMEOUT, DnsTimeout);
-	NetSetHandler(DnsHandler);
+	net_set_udp_handler(DnsHandler);
 
 	DnsSend();
 }