copy-firmware: add usage help text

Add a -h/--help command-line option to show some usage help text.
Also display that usage whenever an error occurs.

Signed-off-by: Timur Tabi <ttabi@nvidia.com>

copy-firmware.sh

@@ -11,8 +11,13 @@ compext=
 destdir=
 num_jobs=1
 
+usage() {
+    echo "Usage: $0 [-v] [-jN] [--xz|--zstd] <destination directory>"
+}
+
 err() {
     printf "ERROR: %s\n" "$*"
+    usage
     exit 1
 }
 
@@ -66,6 +71,11 @@ while test $# -gt 0; do
             shift
             ;;
 
+        -h|--help)
+            usage
+            exit 1
+            ;;
+
         *)
             if test -n "$destdir"; then
                 err "unknown command-line options: $*"