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.
We now have 9 different plugins all using this functionality, and we're
about to add one more. Move this into common code so that all the
plugins are using the same endian and bufsz-safe versions.
