Update mpconfigboard.mk

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

Author: dhalbert

ports/stm/boards/meowbit_v121/mpconfigboard.mk

@@ -14,7 +14,7 @@ MCU_SERIES = F4
 MCU_VARIANT = STM32F401xE
 MCU_PACKAGE = LQFP64
 
-CIRCUITPY_PIXELBUF = 0
+OPTIMIZATION_FLAGS = -Os
 
 LD_COMMON = boards/common_default.ld
 LD_FILE = boards/STM32F401xe_boot.ld