Activate default -Os optimization for Platform.IO (#619)

Author: maxgerhardt

tools/platformio-build.py

@@ -51,6 +51,7 @@
     ASFLAGS=env.get("CCFLAGS", [])[:],
 
     CCFLAGS=[
+        "-Os", # Optimize for size by default
         "-Werror=return-type",
         "-march=armv6-m",
         "-mcpu=cortex-m0plus",