Allow changing PICO_STDIO_USB_CONNECT_WAIT_TIMEOUT_MS from the command line

[peterharperuk]

It's useful to be able to switch to SDIO USB with cmake variables. But you can't set PICO_STDIO_USB_CONNECT_WAIT_TIMEOUT_MS so often miss the first few log lines. Add an option to set this so you don't have to change the code to set it.