stmhal: Fix build issue when MICROPY_PY_THREAD is disabled.

stmhal/main.c

@@ -454,7 +454,9 @@ int main(void) {
     #endif
 
     // basic sub-system init
+    #if MICROPY_PY_THREAD
     pyb_thread_init(&pyb_thread_main);
+    #endif
     pendsv_init();
     led_init();
 #if MICROPY_HW_HAS_SWITCH

stmhal/stm32_it.c

@@ -289,10 +289,12 @@ void SysTick_Handler(void) {
         dma_idle_handler(uwTick);
     }
 
+    #if MICROPY_PY_THREAD
     // signal a thread switch at 4ms=250Hz
     if (pyb_thread_enabled && (uwTick & 0x03) == 0x03) {
         SCB->ICSR = SCB_ICSR_PENDSVSET_Msk;
     }
+    #endif
 }
 
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