There are several subtle bugs in various places in fwupd caused by not treating
user-provided offsets into buffers as unsafe. As fwupd runs as root we have to
assume that all user firmware is evil, and also that devices cannot be trusted.
Make a helper to put all the logic into one place and convert all users.
In many plugins we've wanted to use ->prepare_firmware() to parse the firmware
ahead of ->detach() and ->write_firmware() but this has the limitation that it
can only return a single blob of data.
For many devices, multiple binary blobs are required from one parsed image,
for instance providing signatures, config and data blobs that have to be pushed
to the device in different way.
This also means we parse the firmware *before* we ask the user to detach.
Break the internal FuDevice API to support these firmware types as they become
more popular.
This also allows us to move the Intel HEX and SREC parsing out of the dfu plugin
as they are used by a few plugins now, and resolving symbols between plugins
isn't exactly awesome.
