Fixed NO_UPDATE_REPOS variable usage

Change-Id: I50a2fd73f30321e97c99947b62fb1729870a2c14
diff --git a/functions-common b/functions-common
index 9093952..6ee8031 100644
--- a/functions-common
+++ b/functions-common
@@ -988,7 +988,7 @@
 # Distro-agnostic package installer
 # install_package package [package ...]
 function update_package_repo {
-    if [[ "NO_UPDATE_REPOS" = "True" ]]; then
+    if [[ "$NO_UPDATE_REPOS" = "True" ]]; then
         return 0
     fi