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/bootp.c b/net/bootp.c
index b472051..b36abac 100644
--- a/net/bootp.c
+++ b/net/bootp.c
@@ -669,9 +669,9 @@
 
 #if defined(CONFIG_CMD_DHCP)
 	dhcp_state = SELECTING;
-	NetSetHandler(DhcpHandler);
+	net_set_udp_handler(DhcpHandler);
 #else
-	NetSetHandler(BootpHandler);
+	net_set_udp_handler(BootpHandler);
 #endif
 	NetSendPacket(NetTxPacket, pktlen);
 }