OTA: Add TFTP client mode, expand ota_basic example.

extras/rboot-ota/ota-tftp.h

@@ -1,5 +1,8 @@
 #ifndef _OTA_TFTP_H
 #define _OTA_TFTP_H
+
+#include "lwip/err.h"
+
 /* TFTP Server OTA Support
  *
  * To use, call ota_tftp_init_server() which will start the TFTP server task
@@ -30,6 +33,17 @@
  */
 void ota_tftp_init_server(int listen_port);
 
+/* Attempt to make a TFTP client connection and download the specified filename.
+
+   'timeout' is in milliseconds, and is timeout for any UDP exchange
+   _not_ the entire download.
+
+   Returns 0 on success, LWIP err.h values for errors.
+
+   Does not change the current firmware slot, or reboot.
+ */
+err_t ota_tftp_download(const char *server, int port, const char *filename, int timeout, int ota_slot);
+
 #define TFTP_PORT 69
 
 #endif