Make sure USERNAME doesn't leak in from ENV

diff --git a/grml-live b/grml-live
index d4e27c7..9a1cd94 100755 (executable)
--- a/grml-live
+++ b/grml-live
@@ -120,6 +120,7 @@ BUILD_ONLY=''
 BUILD_DIRTY=''
 BOOTSTRAP_ONLY=''
 HOSTNAME=''
+USERNAME=''
 CONFIGDUMP=''
 
 # don't use colors/escape sequences