control: Support target_path option in configuration file

The target path containing the interface
xyz.openbmc_project.control.FanPwm or
xyz.openbmc_project.control.FanSpeed can be different between
phosphor-hwmon and dbus-sensor.

In phosphor-hwmon, it is "/xyz/openbmc_project/sensors/fan_tach/".
In dbus-sensor, it is "/xyz/openbmc_project/control/fanpwm/"

This commit supports making this path configurable via "target_path".
By default, it is "/xyz/openbmc_project/sensors/fan_tach/".

Signed-off-by: Chau Ly <chaul@amperecomputing.com>
Change-Id: Id799986ff69455b7c7054070ed2ab3b2e2500a6c
diff --git a/control/fan.hpp b/control/fan.hpp
index de0a85c..bb59493 100644
--- a/control/fan.hpp
+++ b/control/fan.hpp
@@ -67,7 +67,7 @@
     std::string _name;
 
     /**
-     * Map of hwmon target sensors to the service providing them
+     * Map of target sensors to the service providing them
      */
     std::map<std::string, std::string> _sensors;