tools: whitespacecheck: use ignore file to easily add expressions

dist/tools/whitespacecheck/check.sh

@@ -6,7 +6,7 @@
 # General Public License v2.1. See the file LICENSE in the top level
 # directory for more details.
 
-NOVENDOR=":!*/vendor/*"
+IGNORE=`awk '{ printf ":!%s ", $0 }' $(dirname "$0")/ignore_list.txt`
 
 # If no branch but an option is given, unset BRANCH.
 # Otherwise, consume this parameter.
@@ -30,14 +30,14 @@ if [ -z "${BRANCH}" ]; then
 fi
 
 git -c core.whitespace="tab-in-indent,tabwidth=4" \
-    diff --check $(git merge-base ${BRANCH} HEAD) -- *.[ch] ${NOVENDOR}
+    diff --check $(git merge-base ${BRANCH} HEAD) -- *.[ch] ${IGNORE}
 
 RESULT=$?
 
 # Git regards any trailing white space except `\n` as an error so `\r` is
 # checked here, too
 git -c core.whitespace="trailing-space" \
-    diff --check $(git merge-base ${BRANCH} HEAD) -- . ${NOVENDOR}
+    diff --check $(git merge-base ${BRANCH} HEAD) -- . ${IGNORE}
 if [ $? -ne 0 ] || [ $RESULT -ne 0 ]
 then
     echo "ERROR: This change introduces new whitespace errors"

dist/tools/whitespacecheck/ignore_list.txt

@@ -0,0 +1 @@
+*/vendor/*