Ensure scripts are set -u

This will hopefully catch bugs where they happen rather than
allowing scripts to continue on and fail later.

Change-Id: Idacd9274415b21db285a198dafff19b1d19a4a68

bin/dib-lint

@@ -66,7 +66,7 @@ for i in $(find elements -type f); do
         fi
     fi
 
-    # Check that all scripts are set -e
+    # Check that all scripts are set -eu
     # NOTE(bnemec): This doesn't verify that the set call occurs high
     # enough in the file to be useful, but hopefully nobody will be
     # sticking set calls at the end of their file to trick us.  And if
@@ -82,6 +82,12 @@ for i in $(find elements -type f); do
                 rc=1
             fi
         fi
+        if ! excluded setu; then
+            if [ -z "$(grep "^set -[^ ]*u" $i)" ]; then
+                echo "ERROR: $i is not set -u"
+                rc=1
+            fi
+        fi
     fi
 done
 exit $rc