Import lwIP contrib rep

... from http://git.savannah.gnu.org/cgit/lwip/lwip-contrib.git/ into contrib/ subdir, STABLE-2_1_0_RELEASE tag
lwIP contrib is now officially frozen
TODO: Fix build

contrib/examples/tftp/tftp_example.h

@@ -0,0 +1,28 @@
+/*
+ * To change this license header, choose License Headers in Project Properties.
+ * To change this template file, choose Tools | Templates
+ * and open the template in the editor.
+ */
+
+/* 
+ * File:   tftp_example.h
+ * Author: dziegel
+ *
+ * Created on February 17, 2018, 3:43 PM
+ */
+
+#ifndef TFTP_EXAMPLE_H
+#define TFTP_EXAMPLE_H
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+void tftp_example_init(void);
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif /* TFTP_EXAMPLE_H */
+