Stage external apidoc tarball to $ONOS_STAGE_ROOT

Change-Id: I24661c002abd54d85d2bd154f47de13d3821d135
diff --git a/tools/build/onos-build-docs b/tools/build/onos-build-docs
index f7d0a0b..bc1d912 100755
--- a/tools/build/onos-build-docs
+++ b/tools/build/onos-build-docs
@@ -37,7 +37,7 @@
 mvn -f aux-external.xml javadoc:aggregate "$@"
 
 cd target && mv site/apidocs $apidocs
-tar zcf $apidocs.tar.gz $apidocs && cp $apidocs.tar.gz /tmp
+tar zcf $apidocs.tar.gz $apidocs && cp $apidocs.tar.gz $ONOS_STAGE_ROOT
 
 cd $ONOS_ROOT/docs
 processPom internal.xml