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
We don't actually need either of the things it provides (looking up in source
and built, and converting to an absolute path) so just replace it with
g_build_filename() instead.
This also has the advantage that it does the right thing on Windows.
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.
