stm32/Makefile: Allow a board to disable float support.

By using "MICROPY_FLOAT_IMPL = none" in its mpconfigboard.mk file.

ports/stm32/Makefile

@@ -91,9 +91,13 @@ CFLAGS += -DMICROPY_HW_VTOR=$(TEXT0_ADDR)
 ifeq ($(MICROPY_FLOAT_IMPL),double)
 CFLAGS += -DMICROPY_FLOAT_IMPL=MICROPY_FLOAT_IMPL_DOUBLE
 else
+ifeq ($(MICROPY_FLOAT_IMPL),none)
+CFLAGS += -DMICROPY_FLOAT_IMPL=MICROPY_FLOAT_IMPL_NONE
+else
 CFLAGS += -DMICROPY_FLOAT_IMPL=MICROPY_FLOAT_IMPL_FLOAT
 CFLAGS += -fsingle-precision-constant -Wdouble-promotion
 endif
+endif
 
 LDFLAGS = -nostdlib -L $(LD_DIR) $(addprefix -T,$(LD_FILES)) -Map=$(@:.elf=.map) --cref
 LDFLAGS += --defsym=_estack_reserve=8