Based on eg 1e6fd9f2b4, it's understood that
the intention for unix builds is that regular builds disable assert, but
the coverage build should set -O0 and enable asserts.
It looks like this didn't work (even before variants were introduced, eg at
v1.11) -- coverage always built with -Os and -DNDEBUG.
This commit makes it possible for variants to have finer-grained control
over COPT flags, and enables assert() and -O0 on coverage builds.
Other variants already match the defaults so they have been updated.
Invoking "make" will still build the standard "micropython" executable, but
other variants are now build using, eg, "make VARIANT=minimal". This
follows how bare-metal ports specify a particular board, and allows running
any make target (eg clean, test) with any variant.
Convenience targets (eg "make coverage") are provided to retain the old
behaviour, at least for now.
See issue #3043.
