Update mpconfigboard.mk
Update mpconfigboard.mk

The stm boards default to `-O2`. Changing to `-Os` brings it back to ~43kB (!) free even for ja.
adafruit/circuitpython,adafruit/circuitpython,adafruit/circuitpython,adafruit/circuitpython,adafruit/circuitpython,adafruit/circuitpython