Add -Wdouble-promotion and -Wfloat-conversion for most ports to ban out
implicit floating point conversions, and add extra Travis builds using
MICROPY_FLOAT_IMPL_FLOAT to uncover warnings which weren't found
previously. For the unix port -Wsign-comparison is added as well but only
there since only clang supports this but gcc doesn't.
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.
When CFLAGS_EXTRA/LDFLAGS_EXTRA (or anything) is set on the command line of
a make invocation then it will completely override any setting or appending
of these variables in the makefile(s). This means builds like the coverage
variant will have their mpconfigvariant.mk settings overridden. Fix this
by using CFLAGS/LDFLAGS exclusively in the makefile(s), reserving the
CFLAGS_EXTRA/LDFLAGS_EXTRA variables for external command-line use only.
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.
