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.
This option affects py/vm.c and py/gc.c and using -Os gets them compiling a
bit smaller, and small firmware is the aim of these two ports. Also,
having these files compiled with -Os on these ports, and -O3 as the default
on other ports, gives a better understanding of code-size changes when
making changes to these files.
When compiled with hard float the system should enable FP access when it
starts or else FP instructions lead to a fault. But this minimal port does
not enable (or use) FP and so, to keep it minimal, switch to use soft
floating point. (This became an issue due to the recent commit
34c04d2319 which saves/restores FP registers
in the NLR state.)
These ports don't need anything from extmod so don't include those files
at all in the build. This speeds up the build by about 10% when building
with a single core.
This is to keep the top-level directory clean, to make it clear what is
core and what is a port, and to allow the repository to grow with new ports
in a sustainable way.
