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.
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.
David Lechner
Damien George