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/tools/releasetools/build_image.py b/tools/releasetools/build_image.py
index 927d89a..d80e7a6 100755
--- a/tools/releasetools/build_image.py
+++ b/tools/releasetools/build_image.py
@@ -325,6 +325,9 @@
   elif mount_point == "vendor":
     copy_prop("vendor_fs_type", "fs_type")
     copy_prop("vendor_size", "partition_size")
+  elif mount_point == "oem":
+    copy_prop("fs_type", "fs_type")
+    copy_prop("oem_size", "partition_size")
 
   return d
 
@@ -363,6 +366,8 @@
     mount_point = "cache"
   elif image_filename == "vendor.img":
     mount_point = "vendor"
+  elif image_filename == "oem.img":
+    mount_point = "oem"
   else:
     print >> sys.stderr, "error: unknown image file name ", image_filename
     exit(1)