use external_updater

generate_config.sh is renamed to post_update.sh to allow it to be
invoked automatically by external_updater. update_libvpx.sh is removed
as it's no longer needed.

Bug: 388605258
Test: TreeHugger
Change-Id: I6d8895f66b5b3fe97d5b4f6e6d6b6f982fc4332a
4 files changed