How about combine release_image and test_image to a single images/ folder?

How about make folder/file name and umpire resource name consistent?

Re #1: in that case we can't easily distinguish which image is which under the same folder. We'll have to match the file name or using other techniques to determine the type of the image.

Re #2: yeah, hungte and I were discussing this yesterday. I think we should:
  - Put the full hash in Umpire resource file name to prevent collisions.
    (Currently we put only 6 characters and collisions happened some times.)
  - Separate resources by folders such as release_images, test_images.
    (But note that I'm using singular forms in bundle but plural forms here because there may
     be more than one images in Umpire resource folder.)

See my draft implementation in https://chromium-review.googlesource.com/#/c/463127/

The goal is to put everything in one single folder so the name can be consistent (release_image, test_image) and each JSON file manages what bundles have.
You can find the type of payload by the prefix of its file name.

Importing and installing that is pretty fast (unless if you unfortunately need to import from compressed tar.xz...) so I think this is basically a good way to follow on.

Made some experiments for partition compression rate (grabbed a random signed recovery image and est image)

release_image_part3: 1.3G, gz: 541M, xz: 436M
test_image_part1: 1.2G, gz: 305M, xz: 217M
test_image_part3: 2.0G, gz: 989M, xz: 854M

XZ is better than I expected.

seems like most of goals are done, or pending on easy bundle creation.