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/bootp.c b/net/bootp.c
index 43466af..b2f8ad4 100644
--- a/net/bootp.c
+++ b/net/bootp.c
@@ -11,8 +11,8 @@
 #include <common.h>
 #include <command.h>
 #include <net.h>
+#include <net/tftp.h>
 #include "bootp.h"
-#include "tftp.h"
 #include "nfs.h"
 #ifdef CONFIG_STATUS_LED
 #include <status_led.h>