Add support for U-Boot legacy image files

Add comparator for U-Boot legacy image files.

U-Boot legacy image files are standard OS boot files (kernel, rootfs, etc) with a 64 byte header appended to the front of the file by mkimage to provide the load address, entrypoint, and a CRC for the contained image. By treating this as a container and setting our extract method to just cut the first 64 bytes off, the rest of diffoscope's tooling can be used to view the differences of the contained files.

The headers are added as byte strings in the added tests so that additional prebuilt binaries do not have to be checked in.