net: tftp: Move tftp.h file from ./net to ./include/net

This change gives the ability to reuse the <tftp.h> header file by other
subsystems (like e.g. dfu).

Without this change compilation error emerges for the legacy update.c file.

Signed-off-by: Lukasz Majewski <l.majewski@majess.pl>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>
diff --git a/net/rarp.c b/net/rarp.c
index 4ce2f37..1fa11b6 100644
--- a/net/rarp.c
+++ b/net/rarp.c
@@ -8,10 +8,10 @@
 #include <common.h>
 #include <command.h>
 #include <net.h>
+#include <net/tftp.h>
 #include "nfs.h"
 #include "bootp.h"
 #include "rarp.h"
-#include "tftp.h"
 
 #define TIMEOUT 5000UL /* Milliseconds before trying BOOTP again */
 #ifndef	CONFIG_NET_RETRY_COUNT