Turn on BOARD_USES_METADATA_PARTITION in mainline

All the targets that inherit mainline seem to use metadata_partition,
So turn on BOARD_USES_METADATA_PARTITION

Bug: 137920314
Bug: 139731063
Test: m && check if there is /metadata
Change-Id: I17cfb5faf76a3c3d6ce1be594274ea58ebc1d1ae
diff --git a/target/board/BoardConfigMainlineCommon.mk b/target/board/BoardConfigMainlineCommon.mk
index 70cfe39..ff6defd 100644
--- a/target/board/BoardConfigMainlineCommon.mk
+++ b/target/board/BoardConfigMainlineCommon.mk
@@ -13,6 +13,10 @@
 TARGET_COPY_OUT_VENDOR := vendor
 TARGET_COPY_OUT_PRODUCT := product
 
+# Creates metadata partition mount point under root for
+# the devices with metadata parition
+BOARD_USES_METADATA_PARTITION := true
+
 BOARD_VNDK_VERSION := current
 
 # Required flag for non-64 bit devices from P.