Set up rules to build oem.img

To build oem.img:
- You must define BOARD_OEMIMAGE_PARTITION_SIZE in your BoardConfig.mk
- The file system type will be the same as system.img and userdata.img.
- To install a module to oem.img, use "LOCAL_OEM_MODULE := true"
- run "make -j48 showcommands oem_image dist". By default it's not
  built.

Bug: 13367676
Change-Id: I1a26d4d0c61b72ecffe60279667b1b3de050780d
diff --git a/core/clear_vars.mk b/core/clear_vars.mk
index 4912e35..9d83de9 100644
--- a/core/clear_vars.mk
+++ b/core/clear_vars.mk
@@ -127,6 +127,7 @@
 LOCAL_PROTO_JAVA_OUTPUT_PARAMS:=
 LOCAL_NO_CRT:=
 LOCAL_PROPRIETARY_MODULE:=
+LOCAL_OEM_MODULE:=
 LOCAL_PRIVILEGED_MODULE:=
 LOCAL_MODULE_OWNER:=
 LOCAL_CTS_TEST_PACKAGE:=