X-FIRMWARE-VERSION gets set to the content of the file referenced by UCI setting "autoupdater.settings.version_file"
If "autoupdater.settings.version_file" is unset X-FIRMWARE-VERSION is
omitted
This new version of the autoupdater is implemented in C instead of Lua,
allowing us to interface with libuclient (HTTP downloads) and libecdsautil
(signature checks) directly instead of spawning external processes,
saving RAM and making error handling more robust.
[Matthias Schiffer: add commit message]