deploy: add --help option

If receiving bad arguments, the help text is also shown, but on stderr.

openwrt/default.nix

@@ -203,18 +203,32 @@ let
                 command scp -Op ${sshOpts} "$@"
               }
 
+              usage() {
+                echo "usage: $(basename "$0") [options]"
+                echo
+                echo "options:"
+                echo "  --help        Show this help"
+                echo "  --reload      Reload/deploy config without rebooting"
+              }
+
               main() {
                 RELOAD_ONLY=false
                 TIMEOUT=${toString rebootTimeout}s
 
                 for arg in "$@"; do
                   case "$arg" in
+                    --help)
+                      usage
+                      exit 0
+                      ;;
                     --reload)
                       RELOAD_ONLY=true
                       TIMEOUT=${toString reloadTimeout}s
                       ;;
                     *)
                       echo "error: unknown argument: $arg" >&2
+                      echo >&2
+                      usage >&2
                       exit 1
                       ;;
                   esac