log: fix log_flush()

The logging thread is usually woken up by a timer, if the timer
period is longer than 10ms, log_flush() can iterate multiple times
needlessly, while waiting for the next timer. Instead wake up the
logging thread to let it flush the queue immediately. Besides a
hard-coded polling period of 10ms is too long for some applications,
make it a Kconfig parameter.

Signed-off-by: Guennadi Liakhovetski <guennadi.liakhovetski@linux.intel.com>