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/base_rules.mk b/core/base_rules.mk
index 8676170..7a1975f 100644
--- a/core/base_rules.mk
+++ b/core/base_rules.mk
@@ -120,6 +120,8 @@
   else
   ifeq (true,$(LOCAL_PROPRIETARY_MODULE))
     partition_tag := _VENDOR
+  else ifeq (true,$(LOCAL_OEM_MODULE))
+    partition_tag := _OEM
   else
     # The definition of should-install-to-system will be different depending
     # on which goal (e.g., sdk or just droid) is being built.