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

.project

.svghmithumbs

.directory
.pytest_cache
.cache
.coverage_full

syntax: regexp
^tests/.*/build$
^exemples/.*/build$
^.idea/.*
syntax: regexp
^.*\.pyc$
syntax: regexp
^.*~$
syntax: regexp
^.*\.swp$

bug_report.*\.txt
i18n/.*.new$
revision

doc/_build
doc/locale

^.*\$py.class$