Support indirections other than just "label"

The current code only allowed the MODE_<type><Num> env var
to be "label" when doing DBus naming indirection.

Add support to allow it to be anything, and then use that value
to do the lookup.

For example, if MODE_temp1 = "foo", then the code will read the
temp1_foo file to find the value to append to LABEL to find the
DBus object name.  So if temp1_foo contained a 42, then the code
will use the LABEL_temp42 var to find the object name.

Tested: Test on the OpenPower OCC device which use 'label' and now
        'function_id' for the indirections.

Change-Id: I1f3115a2d37d008efca74748ac7eff8434d8320a
Signed-off-by: Matt Spinler <spinler@us.ibm.com>
diff --git a/env.cpp b/env.cpp
index 6841987..677f6fb 100644
--- a/env.cpp
+++ b/env.cpp
@@ -52,6 +52,7 @@
 
 std::string getIndirectID(
         std::string path,
+        const std::string& fileSuffix,
         const SensorSet::key_type& sensor)
 {
     std::string content;
@@ -59,7 +60,7 @@
     path.append(sensor.first);
     path.append(sensor.second);
     path.append(1, '_');
-    path.append(hwmon::entry::label);
+    path.append(fileSuffix);
 
     std::ifstream handle(path.c_str());
     if (!handle.fail())