NEW: sync the code of DJI Payload-SDK version 2.2.1 released on January 20 2021.

Signed-off-by: DJI-Martin <DJI-Martin@dji.com>

sample/platform/rtos_freertos/high-speed_transmission_demo/README.txt

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+Payload SDK High-Speed Transmission Function Sample.
+
+Test Environment:
+Hardware:               Manifold 2-C/G
+Serial:                 USB-to-Serial Controller CP210x
+C Compiler:             GCC version 5.4.0 20160609
+CMake Version:          Cmake version 3.5.1
+
+Interaction Serial Configuration:
+Baud Rate:              115200
+
+Application Introduction:
+
+This sample applies to multiprocessor or multi-SoC complex system, such as Payload SDK demo run on a RTOS development
+board and the video stream need be sent by another device's (Manifold 2-C/G) network port. Because the RTOS is difficult
+to deal with video stream. To get the local and remote network info about aircraft, manifold should communicate with
+RTOS development board by usb-to-serial controller. Then manifold can send video stream via network to aircraft and DJI
+Pilot APP/MSDK APP.
+
+====== The steps to send test video stream and data stream ======
+Step 1 : modify related device information, such as device name of network or uart, uart baud rate.
+
+Step 2 : build source file, use following cmd:
+cd source_code_dir/project
+mkdir build
+cd build
+cmake ..
+make
+
+Step 3 : Manifold should connect to Skyport 2.0/XPort via network cable and connect to RTOS development board
+via usb-to-serial controller, make sure the serial pins are connected properly. And please refer to README.txt file of
+RTOS sample (just like, sample/platform/rtos_freertos/psdk_development_board_1.0/README.txt or
+sample/platform/rtos_freertos/stm32f4_eval/README.txt) for interaction UART pins in RTOS system used to connect to
+Manifold
+
+Step 4 : make sure the RTOS environment is ok and enable the camera function to run the RTOS sample
+
+Step 5 : use the root permission to execute this program, use following cmd:
+sudo ./high-speed_transmission_demo
+
+Step 6 : use DJI Pilot APP/MSDK APP to see video stream and receive test data