scripts: gen_gcov_files.py: add argument help text

Convert the description comment at the top of the file to a
documentation string. That string then maybe viewed with the --help
argument. Also, rework a bit the documentation string: remove usage,
since that is provided by the help message of the argparse module.

Signed-off-by: Ruslan Mstoi <ruslan.mstoi@intel.com>

scripts/gen_gcov_files.py

@@ -4,10 +4,10 @@
 #
 # SPDX-License-Identifier: Apache-2.0
 
-# This script will parse the serial console log file and create the required
-# gcda files.
-# Usage python3 ${ZEPHYR_BASE}/scripts/gen_gcov_files.py -i console_output.log
-# Add -v for verbose
+
+"""This script will parse the serial console log file and create the required
+gcda files.
+"""
 
 import argparse
 import os