BUILD: makefile: drop the SMALL_OPTS settings

BUILD: makefile: drop the SMALL_OPTS settings

These settings were appended to the final build CFLAGS and used to
contain a mix of obsolete settings that can equally be passed in one
of the many other variables such as DEFINE or more recently CFLAGS.
Let's just drop the obsolete comment about it, and check if anything
was forced there, then emit a warning suggesting to move that to other
variables such as DEFINE or CFLAGS, so as to be kind to package
maintainers.