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/Makefile b/core/Makefile
index 9739730..480c719 100644
--- a/core/Makefile
+++ b/core/Makefile
@@ -661,6 +661,7 @@
 $(if $(BOARD_CACHEIMAGE_PARTITION_SIZE),$(hide) echo "cache_size=$(BOARD_CACHEIMAGE_PARTITION_SIZE)" >> $(1))
 $(if $(BOARD_VENDORIMAGE_FILE_SYSTEM_TYPE),$(hide) echo "vendor_fs_type=$(BOARD_VENDORIMAGE_FILE_SYSTEM_TYPE)" >> $(1))
 $(if $(BOARD_VENDORIMAGE_PARTITION_SIZE),$(hide) echo "vendor_size=$(BOARD_VENDORIMAGE_PARTITION_SIZE)" >> $(1))
+$(if $(BOARD_OEMIMAGE_PARTITION_SIZE),$(hide) echo "oem_size=$(BOARD_OEMIMAGE_PARTITION_SIZE)" >> $(1))
 $(if $(INTERNAL_USERIMAGES_SPARSE_EXT_FLAG),$(hide) echo "extfs_sparse_flag=$(INTERNAL_USERIMAGES_SPARSE_EXT_FLAG)" >> $(1))
 $(if $(mkyaffs2_extra_flags),$(hide) echo "mkyaffs2_extra_flags=$(mkyaffs2_extra_flags)" >> $(1))
 $(hide) echo "selinux_fc=$(SELINUX_FC)" >> $(1)