logging: backend: notify when process thread finishes one cycle

This adds the bits to allow the process thread to notify
backends when it has finished processing pending log messages.

This allows, for example, flash-based backends to group writing
log messages to storage to limit wear, since log_output_flush()
is called per message. Also the backend does not have to resort
to using timer to periodically flush its buffer to storage,
avoiding waking up the device when there are no messages.
Another use of this is for backends requiring DMA transfers.
Grouping all pending log messages into one DMA transfer is
sometimes preferrable.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>