Update hwmon fan target sysfs entries

Override the default FanSpeed.Target set implementation so when a target
value is written to the FanSpeed interface it is also updated in the
related fan target sysfs file. This sets a particular fan to the given
target speed.

Resolves openbmc/openbmc#962
Resolves openbmc/phosphor-hwmon#1

Change-Id: I867811737269b3f42d2a0dc15b37782a74f147b8
Signed-off-by: Matthew Barth <msbarth@us.ibm.com>
diff --git a/Makefile.am b/Makefile.am
index b7057ad..1ac9033 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -21,6 +21,7 @@
 	sensorset.cpp \
 	mainloop.cpp \
 	sysfs.cpp \
-	env.cpp
+	env.cpp \
+	fan_speed.cpp
 
 SUBDIRS = . test