DOCS: how to specify a device to flash when multiple are connected?

[sacca97]

As per title, cannot find anything in the docs and JLink does not help

[sacca97]

Thanks to #239

--extrajtagcmd "-select usb=your board s/n"

[andrzej-kaczmarek]

@sacca97 you can also use JLINK_SN env variable, i.e. JLINK_SN=123456 newt load xxx - this basically does the same but it's easier to remember