In the BetaFlight 4.6, several changes caused incompatibility of ELRS RX BF Passthrough flashing.

In the recent BF versions,

• Issuing CLI mode should be set by writing single #, not #\r\n (newline automatically added from SerialHelper.write_str()). This PR modified write_str() to allow writing without newline characters.
• BF's UART refactor changes how serial resources are enumerated. (ref: Refactor uart betaflight/betaflight#13585)
  Example)

   serial VCP 1 115200 57600 0 115200
   serial UART1 0 115200 57600 0 115200
   serial UART2 0 115200 57600 0 115200
   serial UART3 0 115200 57600 0 115200
   serial UART4 0 115200 57600 0 115200
   serial UART5 0 115200 57600 0 115200
   serial UART6 64 115200 57600 0 115200
   serial UART7 0 115200 57600 0 115200
   serial UART8 0 115200 57600 0 115200
  

  Previously it was enumerated as index numbers only (0 instead of UART1 ). ELRS BF Passthrough flashing parses those index numbers, but this breaking change prevented go get into passthrough mode. In this PR, I updated the regex expression to recognize the UART\d strings patterns.