librem5-devkit-flash-image: Support flashing releases with separate boot and OS images
In order to be able to flash devkit releases generated after image-builder!88 is merged, the librem5-devkit-flash-image python script must:
- be able to download both the OS and boot images from a single release
- check their integrity using the manifest
- run the necessary uuu scripts to flash them to their correct location
Ideally, it should still be compatible with older images containing a single image.
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