sensor-mon: Create event logs

Create phosphor-logging event logs when the alarms are tripped and
cleared.  The severity will be 'Error' on trip, and 'Informational'
on clear.

The error names will be xyz.openbmc_project.Sensor.Threshold.Error.
    HardShutdownAlarmLow
    HardShutdownAlarmLowClear
    HardShutdownAlarmHigh
    HardShutdownAlarmHighClear

When the shutdown occurs after the delay, the AlarmLow/AlarmHigh events
are sent again.  This is so there is an event log that occurs at the
time of the shutdown, and it says what the problem was that caused it.

Signed-off-by: Matt Spinler <spinler@us.ibm.com>
Change-Id: If36c48e73f4208d781040c8b2c06995f66bebc20
2 files changed