Add warning for hex files

Most flashing tools want hex files so this is an easy mistake to make. Conversion with objcopy is trivial so we don't have to add support for hex. However, we should at least warn the user. The README is quite clear about that but I had only looked at `--help`.

It seems that I'm not the only who made this mistake: MarsTechHAN#2

ch55xtool/ch55xtool.py

@@ -348,7 +348,7 @@ def main():
         '--file',
         type=str,
         default='',
-        help="The target file to be flashed.")
+        help="The target file to be flashed. This must be a binary file (hex files are not supported).")
     parser.add_argument(
         '-r',
         '--reset_after_flash',
@@ -380,6 +380,8 @@ def main():
 
     if args.file != '':
         payload = list(open(args.file, 'rb').read())
+        if args.file.endswith('.hex') or args.file.endswith('.ihx') or payload[0]==58:
+            print("WARNING: This looks like a hex file. This tool only supports binary files.")
         if ret[0] in ['V2.30']:
             ret = __write_key_ch55x_v20(dev, chk_sum)
             if ret is None: