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/tftp.c b/net/tftp.c
index bf32eab..b2e08b4 100644
--- a/net/tftp.c
+++ b/net/tftp.c
@@ -778,7 +778,7 @@
 	TftpTimeoutCountMax = TftpRRQTimeoutCountMax;
 
 	NetSetTimeout(TftpTimeoutMSecs, TftpTimeout);
-	NetSetHandler(TftpHandler);
+	net_set_udp_handler(TftpHandler);
 #ifdef CONFIG_CMD_TFTPPUT
 	net_set_icmp_handler(icmp_handler);
 #endif
@@ -840,7 +840,7 @@
 #endif
 
 	TftpState = STATE_RECV_WRQ;
-	NetSetHandler(TftpHandler);
+	net_set_udp_handler(TftpHandler);
 }
 #endif /* CONFIG_CMD_TFTPSRV */