scripts: file2hex.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.

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

scripts/file2hex.py

@@ -4,9 +4,13 @@
 #
 # SPDX-License-Identifier: Apache-2.0
 
-# This converts a file to a list of hex characters which can then
-# be included to a source file.
-# Optionally, the output can be compressed if needed.
+
+"""Convert a file to a list of hex characters
+
+The list of hex characters can then be included to a source file. Optionally,
+the output can be compressed.
+
+"""
 
 import argparse
 import codecs