Former releases of uD3TN have included support for STM32/FreeRTOS.
Having this support from the first moment on was a good thing in
order to optimize for platforms with limited resources. As this
platform is not used in current uD3TN setups and as it results in quite
some maintenance work, it is removed with this commit. The removed parts
cover the implementation itself, tests, tools, documentation, parts of
the build/make scripts as well as licensing information.

Signed-off-by: Marius Feldmann <marius.feldmann@d3tn.com>