[nRF52840] ble app uart bi-direction test

    基本上是基於ble_app_uart_test的結果繼續往下做, 前一篇是只有output的部份, 這一篇則加上了input, 達到可以雙向溝通的範例.
    
    1. Test Video:

      
   2. Test Code Download Link: (only support keil-mdk 5)
       nRF52840_ble_app_uart_bidirection_test

   3. Softdevice Download Link:
       s140_nrf52_6.0.0_softdevice.hex

   4. Mergehex Tool Download Link:
       mergehex.exe

   5. Code flow:

• Add define for test command:
• "C" command: Output "Hi, Nordic"
• "R" command: Output "Hello, nRF52840"
• "T" command: Reset Counter Value

• Implement "nus_data_handler" function for "C" & "R" & "T" command:

• Implement main loop flow:

  6. Build code and update code flow:
      Please follow ble_app_uart_test step to do.