logger: unconditionally call _writer.notify()
logger: unconditionally call _writer.notify()

The file writer thread could get into a state where it blocked infinitely
on pthread_cond_wait() (or rather until the logging stops).

This is very rare and the following conditions must be met:
- the buffer is almost empty (<4KB filled), so that the writer thread does
  not write anything.
- an fsync call is scheduled (happens once every second)
- the fsync call takes a long time (several 100ms), during which time the
  complete log buffer fills up.

The main thread would then get into dropout state where it does not call
_writer.notify() anymore.

Notifying the writer thread on every loop update of the main thread fixes
that.

It does not increase resource usage.

acfloria/Firmware,krbeverx/Firmware,krbeverx/Firmware,dagar/Firmware,acfloria/Firmware,acfloria/Firmware,dagar/Firmware,PX4/Firmware,dagar/Firmware,PX4/Firmware,dagar/Firmware,acfloria/Firmware,PX4/Firmware,dagar/Firmware,PX4/Firmware,PX4/Firmware,PX4/Firmware,krbeverx/Firmware,krbeverx/Firmware,acfloria/Firmware,krbeverx/Firmware,krbeverx/Firmware,acfloria/Firmware,PX4/Firmware,krbeverx/Firmware,dagar/Firmware,dagar/Firmware,acfloria/Firmware