kernel: Restore CONFIG_MULTITHREADING=n behavior

The prepare_multithreading()/switch_to_main_thread() steps were being
done unconditionally, when with multhreading disabled we want to jump
straight into the main thread on the existing stack.

Needless to say, that doesn't work well.  Fixes zephyrproject-rtos#8361.

Signed-off-by: Andy Ross <andrew.j.ross@intel.com>

kernel/init.c

@@ -280,6 +280,7 @@ static void init_idle_thread(struct k_thread *thr, k_thread_stack_t *stack)
  *
  * @return N/A
  */
+#ifdef CONFIG_MULTITHREADING
 static void prepare_multithreading(struct k_thread *dummy_thread)
 {
 #ifdef CONFIG_ARCH_HAS_CUSTOM_SWAP_TO_MAIN
@@ -377,6 +378,7 @@ static void switch_to_main_thread(void)
 	_Swap(irq_lock());
 #endif
 }
+#endif /* CONFIG_MULTITHREDING */
 
 u32_t z_early_boot_rand32_get(void)
 {
@@ -434,6 +436,7 @@ extern uintptr_t __stack_chk_guard;
  */
 FUNC_NORETURN void _Cstart(void)
 {
+#ifdef CONFIG_MULTITHREADING
 #ifdef CONFIG_ARCH_HAS_CUSTOM_SWAP_TO_MAIN
 	struct k_thread *dummy_thread = NULL;
 #else
@@ -444,6 +447,7 @@ FUNC_NORETURN void _Cstart(void)
 	struct k_thread *dummy_thread = (struct k_thread *)&dummy_thread_memory;
 
 	memset(dummy_thread_memory, 0, sizeof(dummy_thread_memory));
+#endif
 #endif
 	/*
 	 * The interrupt library needs to be initialized early since a series
@@ -466,11 +470,15 @@ FUNC_NORETURN void _Cstart(void)
 	__stack_chk_guard = z_early_boot_rand32_get();
 #endif
 
+#ifdef CONFIG_MULTITHREADING
 	prepare_multithreading(dummy_thread);
-
-	/* display boot banner */
-
 	switch_to_main_thread();
+#else
+	bg_thread_main(NULL, NULL, NULL);
+
+	while (1) {
+	}
+#endif
 
 	/*
 	 * Compiler can't tell that the above routines won't return and issues