OpenPicoRTOS v1.9.9

[jnaulet]

What's new ?

Minor update, support for new architecture:

- STM8: support for STM8Sx003
- STM8S minimal drivers (clock, mux, gpio) + demo, staging

A few fixes have been made to the build system, mainly to accomodate for newer versions of
the gcc compiler, thanks to David J. Fiddes

ATTiny414 machine has been added, courtesy of Platima.

More tests & static analysis have been added in the corresponding directory & code has been
made more compliant with the newer versions of the tools in the meantime.