Allow to override CirrOS version in init-runonce

This is to allow us to customize for CI.

Change-Id: Ie45860fa2228c21ce45ac8118267fb8304129802

tools/init-runonce

@@ -19,7 +19,7 @@ fi
 # to be created.
 
 ARCH=$(uname -m)
-CIRROS_RELEASE=0.6.1
+CIRROS_RELEASE=${CIRROS_RELEASE:-0.6.1}
 IMAGE_PATH=/opt/cache/files/
 IMAGE_URL=https://github.com/cirros-dev/cirros/releases/download/${CIRROS_RELEASE}/
 IMAGE=cirros-${CIRROS_RELEASE}-${ARCH}-disk.img