Openxc

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* Fixed receiving data from new network based VI using null delimiters
* Make sure const variables are initialized in generated firmware code.

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* Support communicating with vi-firmware v6.x.
* Support generating code for vi-firmware v6.x.
* Support control commands over serial in additional to USB (version, device ID
diagnostics, raw CAN messages).
* Read debug log data over USB from a VI (see the `log-mode` flag on the CLI
tools).
* Support diagnostic request/response exchange with a v6.x VI.
* Add `openxc-diag`, `openxc-scanner` and `openxc-obd2scanner` CLI tools.
* Increase Bluetooth VI connection reliabilty when sending commands.

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* In generated code, include the extra sources after messages and signals so
they are already defined.

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* Fix bit_numbering_inverted flag override in code generation.
* Fix using only 1 CAN bus with code generation.

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* Rename and supported 'raw_writable' flag on CAN bus in VI configuration.

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* Dropped support for Python 3 - needed to add protobuf dependency, which
doesn't work with Python 3 yet.
* Significant speedup in VI firmware code generation with simple parsed XML
caching
* Parse binary output payload from a vehicle interface (experimental)
* Small bug fixes and efficiency improvements with code generation.