op-build-env: Don't override BR2_DL_DIR

Don't override BR2_DL_DIR if we have one defined

Signed-off-by: Klaus Heinrich Kiwi <klaus@linux.vnet.ibm.com>
diff --git a/op-build-env b/op-build-env
index 2537c4f..8db36bb 100755
--- a/op-build-env
+++ b/op-build-env
@@ -12,7 +12,7 @@
 fi
 
 export BR2_EXTERNAL=${__PWD}/openpower
-export BR2_DL_DIR=${__PWD}/dl
+if [ -z "${BR2_DL_DIR-}" ]; then export BR2_DL_DIR=${__PWD}/dl; fi
 
 export LANG=en_US.UTF-8