net: tftp: Adding support for TFTP Client.

Adding RFC1350 compliant support for TFTP Client in Zephyr. The
current implementation is minimal and only supports the ability
to get a file from the server.

Things for the future include support for putting files to
server and adding support for RFC2347.

Signed-off-by: Bilal Wasim <bilalwasim676@gmail.com>

include/net/tftp.h

@@ -0,0 +1,50 @@
+/*
+ * Copyright (c) 2020 InnBlue
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+/** @file tftp.h
+ *
+ *  @brief Zephyr TFTP Implementation
+ */
+
+#ifndef ZEPHYR_INCLUDE_NET_TFTP_H_
+#define ZEPHYR_INCLUDE_NET_TFTP_H_
+
+#include <zephyr.h>
+#include <net/socket.h>
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+struct tftpc {
+	u8_t   *user_buf;
+	u32_t  user_buf_size;
+};
+
+/* TFTP Client Error codes. */
+#define TFTPC_SUCCESS             0
+#define TFTPC_DUPLICATE_DATA     -1
+#define TFTPC_BUFFER_OVERFLOW    -2
+#define TFTPC_UNKNOWN_FAILURE    -3
+#define TFTPC_REMOTE_ERROR       -4
+#define TFTPC_RETRIES_EXHAUSTED  -5
+
+/* @brief This function gets "file" from the remote server.
+ *
+ * @param server      Control Block that represents the remote server.
+ * @param client      Client Buffer Information.
+ * @param remote_file Name of the remote file to get.
+ * @param mode        TFTP Client "mode" setting
+ *
+ * @return TFTPC_SUCCESS if the operation completed successfully.
+ *         TFTPC_BUFFER_OVERFLOW if the file is larger than the user buffer.
+ *         TFTPC_REMOTE_ERROR if the server failed to process our request.
+ *         TFTPC_RETRIES_EXHAUSTED if the client timed out waiting for server.
+ */
+int tftp_get(struct sockaddr *server, struct tftpc *client,
+	     const char *remote_file, const char *mode);
+
+#endif /* ZEPHYR_INCLUDE_NET_TFTP_H_ */