Merge branch 'feature/release_build' into 'master'

Merge branch 'feature/release_build' into 'master'

Add menuconfig setting for optimization level

This change adds two options (Debug/Release) for optimization level.
- Debug (default) enables -O0
- Release enables -Os and adds `-DNDEBUG`, which removes all assert() statements

These options may be overriden at project level by adding necessary flags to CFLAGS/CXXFLAGS.

Debugging symbols are kept in both cases, although we may add an option to strip output file if necessary.

Also we used to define all common compiler flags in CPPFLAGS, and then prepended them to CFLAGS/CXXFLAGS.
It made it impossible to add preprocessor macros to CPPFLAGS at component level (one had to use CFLAGS/CXXFLAGS instead).
Some third party libraries are not compatible with this approach. Changed to the more common way of using these variables.

CI will build both debug (default) and release.

See merge request !138