Useless TIOCSERGSTRUCT logic in most serial drivers is not compiled unless CONFIG_DEBUG and CONFIG_SERIAL_TIOCSERGSTRUCT are defined

Various Kconfig files still have references to CONFIG_ variables.  Some in harmless comments, some in config definionts which is not harmless.  All removed

Fix major misthink in Cortex-M0 port:  The Cortex-M0 has no BASEPRI register.  We have to revert to using the nasty PRIMASK register