Support writing binary images directly from flash command.

[AVee]

Currently the flash command expects an ELF image to be flashed to the device. This makes sense in the context of a cargo project since the output there will always be an ELF image. But on the command line it would be useful to be able to directly flash an already converted image. In our case we already build those for OTA updates and it would be convenient to be able to use the same files when flashing directly.

It looks to me like it's should be doable to just detect the type of file passed to the flash command based on the magic bytes in the file. An ELF file could be converted and flashed, but when a ESP binary is found it's just flashed directly. All of that would then be transparent for the user and 'just work' regardless of the type of image used. Would this be a sensible approach, or would a more explicit (separate command or extra argument) be preferred?

This might be a solution to #684 as well.

[AVee]

I made a working POC by using code from the image_format module to implement the FirmwareImage trait. That works, but does a lot of needless work splitting the file into sectors and then merging them back again the exact same way.

@jessebraham A clean solution for this would involve some refactoring of that code, probably in a way where a lot of handling moves into the FirmwareImage trait and it implementers. I'd be willing to give that a go, but I don't know if you have anything planned for #772 in that area.