Correctly attach into the alternate mode after the update has completed.
The vendor was appending two files to make LVFS distribution 'easier' but I'd
much rather use the same deliverables as Windows. This also allows us to
simplify the firmware loading.
These are visually similar to Intel hex files, but different enough to demand
their own parser. Multiple images can be stored in one firmware file, with the
`addr` set to the SiliconID and the `idx` set to the position in the file.
