Fix Makefiles: -O3 option was not being applied; mkdir for STM.

py/py.mk

@@ -101,8 +101,8 @@ $(PY_BUILD)/emitnthumb.o: py/emitnative.c
 	$(call compile_c)
 
 # optimising gc for speed; 5ms down to 4ms on pybv2
-$(PY_BUILD)gc.o: CFLAGS += -O3
+$(PY_BUILD)/gc.o: CFLAGS += -O3
 
 # optimising vm for speed, adds only a small amount to code size but makes a huge difference to speed (20% faster)
-$(PY_BUILD)vm.o: CFLAGS += -O3
+$(PY_BUILD)/vm.o: CFLAGS += -O3
 

stm/Makefile

@@ -151,7 +151,7 @@ OBJ += $(addprefix $(BUILD)/, $(SRC_STMUSBD:.c=.o))
 OBJ += $(addprefix $(BUILD)/, $(SRC_FATFS:.c=.o))
 #OBJ += $(addprefix $(BUILD)/, $(SRC_CC3K:.c=.o))
 
-all: $(BUILD) $(BUILD)/flash.dfu
+all: $(BUILD)/flash.dfu
 
 $(BUILD)/flash.dfu: $(BUILD)/flash0.bin $(BUILD)/flash1.bin
 	$(ECHO) "Create $@"