util/lint: Fix linter for old license headers

Somehow I lost a $ on headerlist between when I tested the script
and when I submitted it, turning headerlist into plain text instead
of a variable name.  This makes the test always pass.
- Fix variable bug.
- exclude this script from the check.
- update test for empty HEADER_DIRS variable.

Change-Id: I6080c520bc741e9d689f7c66ee97879afc8ba38c
Signed-off-by: Martin Roth <martinroth@google.com>
Reviewed-on: https://review.coreboot.org/13057
Tested-by: build bot (Jenkins)
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Patrick Georgi <pgeorgi@google.com>

util/lint/lint-stable-009-old-licenses

@@ -29,10 +29,10 @@ HEADER_EXCLUDED="\
 _shipped$|\
 ^util/scripts/no-fsf-addresses.sh|\
 ^util/lint/lint-000-license-headers|\
-^util/lint/lint-stable-000-license-headers\
+^util/lint/lint-stable-009-old-licenses\
 "
 
-if [ "$HEADER_DIRS" = "" ]; then
+if [ -z "$HEADER_DIRS" ]; then
 	HEADER_DIRS="src util"
 fi
 
@@ -42,7 +42,7 @@ headerlist=$(git ls-files $HEADER_DIRS | egrep -v "($HEADER_EXCLUDED)")
 
 #check for the old style header
 headerlist=$(grep -il "You should have received a copy of the GNU" \
-	headerlist 2>/dev/null)
+	$headerlist 2>/dev/null)
 
 for file in $headerlist; do
 	echo "$file has the old GPL header."