scripts: Fix toolchain_boards argument parsing

This is similar to --toolchain-boards parsing crrev.com/c/4857289 but
fixes the will be deprecated argument.

BUG=None
TEST=CQ

Change-Id: I4c3d9359e7b471800c2fda899e03034c1a5b9395
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/chromite/+/4895390
Commit-Queue: Po-Hsien Wang <pwang@chromium.org>
Auto-Submit: Po-Hsien Wang <pwang@chromium.org>
Reviewed-by: Alex Klein <saklein@chromium.org>
Tested-by: Po-Hsien Wang <pwang@chromium.org>
diff --git a/scripts/update_chroot.py b/scripts/update_chroot.py
index e09371f..51903c6 100644
--- a/scripts/update_chroot.py
+++ b/scripts/update_chroot.py
@@ -83,6 +83,7 @@
     )
     group.add_argument(
         "--toolchain_boards",
+        nargs="+",
         deprecated=deprecated % "--toolchain-boards",
         help=argparse.SUPPRESS,
     )