bare-arm, minimal: Set CSUPEROPT=-Os to get minimal firmware size.

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.

ports/minimal/Makefile

@@ -28,6 +28,8 @@ CFLAGS = -m32 $(INC) -Wall -Werror -std=c99 $(COPT)
 LDFLAGS = -m32 -Wl,-Map=$@.map,--cref -Wl,--gc-sections
 endif
 
+CSUPEROPT = -Os # save some code space
+
 # Tune for Debugging or Optimization
 ifeq ($(DEBUG), 1)
 CFLAGS += -O0 -ggdb