px_uploader.py: write timeout workaround

This is a workaround for the write timeout that we have seen for some
host computers trying to flash the firmware.

We don't know the root cause of the problem but we do observed the
following:

- For blocking writes with timeout (Pyserial write_timeout=0.5):
  write() throws SerialTimeoutException. In systrace we see that the
  select() call after write waiting for the write to be finished hangs
  and finally times out.
- For blocking writes without timeout (Pyserial write_timeout=None):
  write() hangs indefinitely. In systrace we see that the
  select() call after write waiting for the write to be finished hangs.
- For non-blocking writes:
  write() works but flush() hangs. In systrace we see that
  ioctl(fd, TCSBRK, 1) which is (correctly) triggered by termios tcdrain
  hangs.

Inspecting USB traffic using usbmon, we can see that the data which is
written actually seems to be sent and looking at responses from the
Pixhawk bootloader and the timings it looks like all the data has
arrived.

This workaround uses non-blocking writes without flushing and this
seemed to prevent the issue from happening so far.

Debugging was done in collaboration with Beat Küng and David Sidrane.