rootdir: make sure the /oem mountpoint is always available

The /oem mount point is used to mount semi-trusted data, and
many Android One devices depend on it. Make sure it's guaranteed
to always be available.

Bug: 20816563
Change-Id: Ib5272f025d14d4da6125d753879054b3faeae696
diff --git a/rootdir/Android.mk b/rootdir/Android.mk
index 3ecb1db..7ab76b8 100644
--- a/rootdir/Android.mk
+++ b/rootdir/Android.mk
@@ -26,7 +26,7 @@
 #
 # create some directories (some are mount points)
 LOCAL_POST_INSTALL_CMD := mkdir -p $(addprefix $(TARGET_ROOT_OUT)/, \
-    sbin dev proc sys system data)
+    sbin dev proc sys system data oem)
 
 include $(BUILD_SYSTEM)/base_rules.mk