dist/tools/uncrustify/uncrustify.sh: fix uncrustify check logic

Previously, uncrustify.sh would fail (report uncrustifying necessary) if
there was any output of uncrustify. Turns out uncrustify sometimes
outputs something.

This commit changes the logic to use uncrustify's output value as
indicator.

Also, adds printing which file causes the check to fail.

dist/tools/uncrustify/uncrustify.sh

@@ -15,11 +15,12 @@ FILES=$(changed_files | grep -xf "$WHITELIST" | grep -xvf "$BLACKLIST")
 check () {
     for F in $FILES
     do
-        OUT="$(uncrustify -c "$UNCRUSTIFY_CFG" -f "$RIOTBASE/$F" --check 2> /dev/null)"
-        if [ "$OUT" ] ; then
+        uncrustify -c "$UNCRUSTIFY_CFG" -f "$RIOTBASE/$F" \
+            --check > /dev/null 2>&1 || {
+            echo "file $F needs to be uncrustified."
             echo "Please run 'dist/tools/uncrustify/uncrustify.sh'"
             exit 1
-        fi
+        }
     done
     echo "All files are uncrustified!"
 }