logging: log_core: support CONFIG_LOG_PROCESS_TRIGGER_THRESHOLD value of 1

Previous implementation didn't work if CONFIG_LOG_PROCESS_TRIGGER_THRESHOLD
was set to 1. Minmimum value that worked was 2. A value of 1 would
just be ignored and act like 0 with threshold triggerring disabled.

Signed-off-by: Mike J. Chen <mjchen@google.com>