repo: Use imx-8 dunfell branch

Commit might have been relocated, let's use imx's default dunfell branch in main
I'll update the pinned SHA1 in dunfell branch

Relate-to: https://booting.oniroproject.org/rzr/meta-oniro-blueprint-flutter/-/issues/5


Signed-off-by: Philippe Coval <philippe.coval.ext@huawei.com>