[fuchsia] Correctly download boot images to peer directories

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}

build/fuchsia/update_images.py

@@ -48,12 +48,12 @@ def GetAllImages(boot_image_names):
   return images_to_download
 
 
-def DownloadSdkBootImages(bucket, sdk_hash, boot_image_names, image_output_dir):
+def DownloadSdkBootImages(bucket, sdk_hash, boot_image_names, image_root_dir):
   images_to_download = GetAllImages(boot_image_names)
   for image_to_download in images_to_download:
     device_type = image_to_download[0]
     arch = image_to_download[1]
-    image_output_dir = os.path.join(image_output_dir, arch, device_type)
+    image_output_dir = os.path.join(image_root_dir, arch, device_type)
     if os.path.exists(image_output_dir):
       continue