[chore] add 702 dualuart firmware

tools/dualuart_firmware/README.md

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+# Bouffalo DualUart
+
+This is the instructions for the dualuart firmware used with BL702 or BL616.
+
+## Hardware
+
+## Sipeed RV-Debugger-Plus
+
+- Use BL702C-A0
+
+![RV-Debugger-Plus](rv-debugger.png)
+
+## Flash download
+
+- Open BL Devcube
+- Select IOT UI and select single download
+- Load firmware
+- Press and hold the boot pin then plug the usb in the computer to go to the boot mode.
+- Download
+
+![Flash download](flash_download.png)
+
+## UART PIN
+
+**DTR** and **RTS** are only used for downloading with devcube.
+
+|   Function    |  Label   |  GPIO  |
+|:-------------:|:--------:|:------:|
+|UART0 TX       | TCK   | GPIO15  |
+|UART0 RX       | TDI   | GPIO0  |
+|UART0 DTR      | TMS   | GPIO2  |
+|UART0 RTS      | TDO   | GPIO1  |
+|UART1 TX       | TX   | GPIO26  |
+|UART1 RX       | RX   | GPIO27  |
+|UART1 DTR      | DTR   | GPIO28  |
+|UART1 RTS      | RTS   | GPIO24  |