librem5-devkit-flash-image: Add multi-images support
requested to merge Bonstra/librem5-devkit-tools:separate-boot-and-os-images-flasher-support into master
Add support for downloading and flashing of images with distinct boot/OS image files.
The goal is to be able to download and flash the multiple images
produced by image-builder with the want_boot_image set to 1, while
still retaining compatibility previous single-file images.
See image-builder!88 and #10
The manifest is downloaded first to determine the image type (single or multi). For multi-files images, the additional boot image is downloaded and verified unsing the metadata from the manifest, and flashed using a specific UUU script.