This allows us to replace the 'dfu-tool replace' with a much more
controlled 'fwupdtool firmware-patch' command that patches at a set of
specific offsets.
Drop in the new functionality into the existing firmware ->write()
vfuncs so that the image headers, footers and/or checksums are correct.
This makes a lot more sense; we can parse a firmware and export the same XML
we would use in a .builder.xml file. This allows us to two two things:
* Check we can round trip from XML -> binary -> XML
* Using a .builder.xml file we can check ->write() is endian safe
This allows us to 'nest' firmware formats, and removes a ton of duplication.
The aim here is to deprecate FuFirmwareImage -- it's almost always acting
as a 'child' FuFirmware instance, and even copies most of the vfuncs to allow
custom types. If I'm struggling to work out what should be a FuFirmware and
what should be a FuFirmwareImage then a plugin author has no hope.
For simple payloads we were adding bytes into an image and then the image into
a firmware. This gets really messy when most plugins are treating the FuFirmware
*as* the binary firmware file.
The GBytes saved in the FuFirmware would be considered the payload with the
aim of not using FuFirmwareImage in the single-image case.
