Fix issues and support signed sensor values

Sensor will get "disable" when the command "ipmitool sdr elist" is
executed that if sensorReadingType is 0x6F.

sensor_units_1 is always set to 0 currently. To support the display of
signed sensor values, we add the attribute "sensorUnits1" to the sensor
mapping yaml. This attribute can be used to determine whether the
sensor is signed.

It were making negative values 0 in get::readingData(). Fix the issue
by using a int32_t and add an overflow check.

Change-Id: I705defcf18805db9ada7d0de0738a59aedab61df
Signed-off-by: Tony Lee <tony.lee@quantatw.com>
6 files changed