[mbed] ERROR: Unable to reset the target board connected to your system.

I'm getting
```
$ mbed compile --flash
...
[mbed] Detected "NRF52840_DK" connected to "/media/mark/MBED" and using com port "/dev/ttyACM0"
[mbed] ERROR: Unable to reset the target board connected to your system.
[mbed] ERROR: This might be caused by an old interface firmware.
[mbed] ERROR: Please check the board page for new firmware.
```
with return code 1, because `mbed_sterm` has no way to return true if `sterm` is false.
https://github.com/ARMmbed/mbed-cli/blob/master/mbed/mbed.py#L1785

I think just setting `result` to true if `sterm` is false is fine? I did it locally and it fixed my issue, but I didn't check all the places it's called.