thermal: add the support for building the generic thermal as a module

Build the generic thermal driver as module "thermal_sys".

Make ACPI thermal, video, processor and fan SELECT the generic
thermal driver, as these drivers rely on it to build the sysfs I/F.

Signed-off-by: Zhang Rui <rui.zhang@intel.com>
Acked-by: Jean Delvare <khali@linux-fr.org>
Signed-off-by: Len Brown <len.brown@intel.com>
diff --git a/drivers/thermal/Makefile b/drivers/thermal/Makefile
index 8ef1232..02b64517 100644
--- a/drivers/thermal/Makefile
+++ b/drivers/thermal/Makefile
@@ -2,4 +2,5 @@
 # Makefile for sensor chip drivers.
 #
 
-obj-$(CONFIG_THERMAL)		+= thermal.o
+thermal_sys-objs	+= thermal.o
+obj-$(CONFIG_THERMAL)		+= thermal_sys.o