Some parsers are ignoring the magic when using _FLAG_IGNORE_CHECKSUM
(which is wrong; fuzzers have no problem with enforcing a static prefix)
and other either disregard the offset or check the magic in an unsafe
way. Also, use FWUPD_ERROR_INVALID_FILE consistently for magic failure.
Add a vfunc, and move all the clever code into one place.
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.
