use defines in build_flags (CPPDEFINES) to determine SPL core files inclusion

[devegied]

• Framework: Standard Peripheral Library (SPL)

When SDCC is used to determine from stm8s_conf.h what SPL modules should be compiled, all defines from build_flags are ignored. So there is no way to control SPL included modules from platformio.ini

Please change this part of builder to take note of build_flags.