MQTT: add JSON payload corresponding to IEC Structured types
- uses https://github.com/cesanta/frozen
- frozen.c is added as a resulting c file in generat_C (fix toolchain_gcc.py as it was breaking dependency checking)
- structured types are converted to C #define, in turn generating json_scanf and json_printf statements
Overview: IDE and runtime(s)
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TODO: take content from https://hg.beremiz.org/beremiz_docs/