ports/stm32: Fix build with Zephyr threading disabled.

When MICROPY_ZEPHYR_THREADING=0 is passed on the command line, the build
would fail because the preprocessor symbol was not defined. The board
config header unconditionally defined it as 1, which then tried to
include Zephyr headers that weren't available.

Fix by:
- Making the board config define conditional (#ifndef guard)
- Adding an else clause in the Makefile to define the symbol as 0 when
  the feature is disabled

Author: pi-anl

ports/stm32/Makefile

@@ -124,6 +124,9 @@ $(ZEPHYR_PSP_SWITCH_O): $(TOP)/extmod/zephyr_kernel/zephyr_psp_switch.S
 	$(ECHO) "CC $<"
 	$(Q)$(CC) $(ZEPHYR_AS_FLAGS) -c -o $@ $<
 
+else
+# Zephyr threading disabled - ensure preprocessor symbol is defined as 0
+CFLAGS += -DMICROPY_ZEPHYR_THREADING=0
 endif
 
 CROSS_COMPILE ?= arm-none-eabi-

ports/stm32/boards/NUCLEO_F429ZI/mpconfigboard.h

@@ -1,8 +1,10 @@
 #define MICROPY_HW_BOARD_NAME       "NUCLEO-F429ZI"
 #define MICROPY_HW_MCU_NAME         "STM32F429"
 
-// Enable Zephyr threading for development
+// Enable Zephyr threading for development (can be overridden via CFLAGS)
+#ifndef MICROPY_ZEPHYR_THREADING
 #define MICROPY_ZEPHYR_THREADING    1
+#endif
 
 #define MICROPY_HW_HAS_SWITCH       (1)
 #define MICROPY_HW_HAS_FLASH        (1)