scripts/check-config.sh: Add usage()

When arguments are not supplied the error message is misleading and
doesn't tell what exactly has to be done.

Fix this by adding usage() and call it if above circumstance occurs.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Simon Glass <sjg@chromium.org>

scripts/check-config.sh

@@ -17,6 +17,15 @@
 set -e
 set -u
 
+PROG_NAME="${0##*/}"
+
+usage() {
+	echo "$PROG_NAME <path to u-boot.cfg> <path to whitelist file> <source dir>"
+	exit 1
+}
+
+[ $# -ge 3 ] || usage
+
 path="$1"
 whitelist="$2"
 srctree="$3"