match thresholdAsserted signal for threshold event monitor

thresholdAsserted signal includes the sensor reading that caused
threshold property change. This removes the need to read the
sensor value at the time of logging, which may miss the bad reading
when the log is delayed.

Tested:
Build and send the following command
busctl set-property xyz.openbmc_project.ADCSensor
/xyz/openbmc_project/sensors/voltage/P1V8_PCH
xyz.openbmc_project.Sensor.Value Value d 1.97

observed the following event logged
P1V8_PCH sensor crossed a critical high threshold going high. Reading=1.970000 Threshold=1.961000.

Signed-off-by: Zhikui Ren <zhikui.ren@intel.com>
Change-Id: Ie8831a9ba48188e1dd343db22583485c02f35dbc
diff --git a/include/sel_logger.hpp b/include/sel_logger.hpp
index 58c7cc6..35f8b95 100644
--- a/include/sel_logger.hpp
+++ b/include/sel_logger.hpp
@@ -32,6 +32,7 @@
     std::numeric_limits<uint16_t>::max();
 static constexpr size_t selEvtDataMaxSize = 3;
 static constexpr size_t selOemDataMaxSize = 13;
+static constexpr uint8_t selEvtDataUnspecified = 0xFF;
 
 static const std::filesystem::path selLogDir = "/var/log";
 static const std::string selLogFilename = "ipmi_sel";