system/dd: implement "--help" parameter

Previously the usage hints were displayed only in case of errors.

Signed-off-by: Lars Kruse <devel@sumpfralle.de>

Author: sumpfralle

system/dd/dd_main.c

@@ -309,6 +309,7 @@ int main(int argc, FAR char **argv)
   uint32_t sector = 0;
   int ret = ERROR;
   int i;
+  bool show_help = false;
 
   /* Initialize the dd structure */
 
@@ -379,13 +380,25 @@ int main(int argc, FAR char **argv)
               cur = next + 1;
             }
         }
+      else if (strcmp(argv[i], "--help") == 0)
+        {
+          show_help = true;
+        }
       else
         {
           print_usage(stderr);
           goto errout_with_paths;
         }
     }
 
+  /* Help requested? Emit usage hints and exit. */
+
+  if (show_help)
+    {
+      print_usage(stdout);
+      return 0;
+    }
+
   /* If verify enabled, infile and outfile are mandatory */
 
   if ((dd.oflags & O_RDONLY) && (infile == NULL || outfile == NULL))