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>

resources/scripts/download/memtest86plus

@@ -23,6 +23,23 @@
 [ "x${DEBUG+set}" = 'xset' ] && set -v
 set -u -e
 
+usage()
+{
+	progname="./download memtest86plus"
+	printf "Usage:\n"
+	printf "\t%s         # %s\n" \
+	       "${progname}" \
+	       "Download MemTest86+"
+	printf "\t%s --help  # %s\n" \
+	       "${progname}" \
+	       "Prints this help"
+}
+
+if [ $# -ne 0 ] ; then
+	usage
+	exit 0
+fi
+
 # Get the last version of MemTest86+ used, apply patches, build it.
 
 # Remove the old version that may exist