flash-image: Print defaults

This makes it simpler for people who found --help

scripts/librem5-flash-image

@@ -58,6 +58,7 @@ FB: flash -raw2sparse all {image}
 FB: Done
 '''
 BLOCK_SIZE = 8192
+RETRIES = 10
 
 
 class VerifyImageException(Exception):
@@ -266,13 +267,15 @@ def main():
                         help='Skip temporary directory cleanup')
     parser.add_argument('--skip-flash', action='store_true', default=False,
                         help='Do all the preparations but don\'t flash')
-    parser.add_argument('--download-attempts', type=int, default=10,
+    parser.add_argument('--download-attempts', type=int, default=RETRIES,
                         help="Maximum number of attempts to resume "
-                        "devkit image download. 0-unlimited")
+                        "image download. 0: unlimited, default is {}".format(RETRIES))
     parser.add_argument('--variant', choices=['legacy', 'current', 'next', 'plain'], default=BOARD_VARIANT,
-                        help='Variant of the board to download ( legacy, current, next )')
+                        help="Variant of the board to download ( legacy, current, next ), "
+                        "default is '{}'".format(BOARD_VARIANT))
     parser.add_argument('--board', choices=['devkit', 'librem5r2', 'librem5r3'], default=BOARD_TYPE,
-                        help='Type of the board to download ( devkit, librem5r2, librem5r3 )')
+                        help="Type of the board to download ( devkit, librem5r2, librem5r3 ) "
+                        "default is '{}'".format(BOARD_TYPE))
 
     group = parser.add_argument_group(title='Testing and debugging options')
     group.add_argument('--debug', action="store_true", default=False,