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Previously, the relative paths for multiple boot images were appended.
For example, for the default `'checkout_fuchsia_boot_images':
"qemu.x64,qemu.arm64"`, the latter's path would be
//third_party/fuchsia-sdk/images/x64/qemu/arm64/qemu/.

Change-Id: Id89c58e0e3a10beaabc0f84278861a0412568d38
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2466837Reviewed-by: Kevin Marshall <kmarshall@chromium.org>
Commit-Queue: David Dorwin <ddorwin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#816416}