[opt][uart_download] Support get download baud rate form SIFLI_DOWNLOAD_BAUD_RATE environment variable.

[HardFatal]

To increase uart download speed, when generate uart_download.bat/uart_download.sh read SIFLI_DOWNLOAD_BAUD_RATE and set to sftool.

useage:

Linux/Mac

export SIFLI_DOWNLOAD_BAUD_RATE=3000000

powershell

$env:SIFLI_DOWNLOAD_BAUD_RATE=3000000

Windows CMD

set SIFLI_DOWNLOAD_BAUD_RATE=3000000

UART 波特率	传输速度
3M	188.85 KiB/s
2M	139.76 KiB/s
1M	85.09 KiB/s

[CLAassistant]

All committers have signed the CLA.

[HalfSweet]

That's a great idea. In addition to the generated uart_download script, could you add the same functionality to the flash command in sdk.py?

[HardFatal]

@HalfSweet

Added the ability to retrieve the baud rate environment variable when executing the sdk.py flash command.

export SIFLI_DOWNLOAD_BAUD_RATE=3000000 download speed at 206.84 KiB/s