mavlink_shell: fix stall on CubeOrange

[julianoes]

On CubeOrange where no console is configured by default, starting MAVLink shell just stalls, and doesn't work.

Also, logfile download has been reported not to work, and again, seems to work with this change.

@bkueng does this make any sense to you? Why are the fflushs required?

Related to #19831.

[dagar]

Looks like the fflush was added here as a potential fix for a crash. #17065

[dagar]

FYI @davids5

[julianoes]

Thinking more about this, fflush would probably make sense but it also makes sense that it stalls if there is nothing to flush to.

[davids5]

@julianoes Is the logic to redirect the console to dev/null in place on this board? If so the question I would have is what does a write to null cause a hang?

[julianoes]

Oh, how do you make it redirect to /dev/null? I don't think it does that. It's just that there is no UARTx_SERIAL_CONSOLE=y defined.

[julianoes]

@davids5 I found it. Redirection to /dev/null is set up but it happens later when the shell thread is started, here:

PX4-Autopilot/src/modules/mavlink/mavlink_shell.cpp

Line 189 in 34c57cc

nsh_consolemain(0, NULL);