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Create --internal-images flag. When used, the update_sdk script will download astro images to //third_party/fuchsia-internal-images.
Include internal images in test target if target_cpu is arm64 and x64 host tools are needed.

Bug: 1134108
Change-Id: If323130add41543e7a7a7f50acbfea80c8ed790b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2446832
Commit-Queue: Chong Gu <chonggu@google.com>
Reviewed-by: Kevin Marshall <kmarshall@chromium.org>
Auto-Submit: Chong Gu <chonggu@google.com>
Cr-Commit-Position: refs/heads/master@{#814047}