download: Add --help in the individual download scripts

This doesn't change the existing usage of the scripts:
- For the Coreboot script, before this change, all arguments that were
  passed were considered as board to download the Coreboot source code
  for.

  Here we added the '--help' and '--list-boards' arguments, so it
  should not be an issue as it is extremely unlikely that a board
  would be called '--help' or '--list-boards'.

- All the other scripts don't use any arguments so passing --help
  should not conflict with the existing usage.

Signed-off-by: Denis 'GNUtoo' Carikli <GNUtoo@cyberdimension.org>

download

@@ -60,6 +60,9 @@ help() {
 	coreboot trees by default, but './download coreboot x60' will only download
 	the coreboot tree required for the target: x60
 
+	Each program download script should also accept the --help paramater to
+	display the usage of the script.
+
 	Refer to the documentation for more information.
 	EOF
 }