diff --git a/config.mk b/config.mk
index c49b350e9..44316b6fd 100644
--- a/config.mk
+++ b/config.mk
@@ -42,9 +42,9 @@ CENTOS_MAJOR?=7
 CENTOS_RELEASE:=$(CENTOS_MAJOR)
 CENTOS_ARCH:=x86_64
 
-UBUNTU_RELEASE:=trusty
-UBUNTU_MAJOR:=14
-UBUNTU_MINOR:=04
+UBUNTU_RELEASE?=trusty
+UBUNTU_MAJOR?=14
+UBUNTU_MINOR?=04
 UBUNTU_RELEASE_NUMBER:=$(UBUNTU_MAJOR).$(UBUNTU_MINOR)
 UBUNTU_KERNEL_FLAVOR?=lts-trusty
 UBUNTU_NETBOOT_FLAVOR?=netboot