方法一:
common/main.c

void main_loop(void)
{
	const char *s;

	bootstage_mark_name(BOOTSTAGE_ID_MAIN_LOOP, "main_loop");

	if (IS_ENABLED(CONFIG_VERSION_VARIABLE))
		env_set("ver", version_string);  /* set version variable */

	cli_init();
	
	
	s = env_get("bootcmd");
	printf("  cli_init  env_get bootcmd is %s  run_command_list\r\n",s);
	run_command_list(s, -1, 0);    // 启动内核

	if (IS_ENABLED(CONFIG_USE_PREBOOT))
		run_preboot_environment_command();

	if (IS_ENABLED(CONFIG_UPDATE_TFTP))
		update_tftp(0UL, NULL, NULL);

	s = bootdelay_process();
	if (cli_process_fdt(&s))
		cli_secure_boot_cmd(s);

	autoboot_command(s);

	cli_loop();
	panic("No CLI available");
}

方法二:
common/autoboot.c

void autoboot_command(const char *s)
{
	debug("### main_loop: bootcmd=\"%s\"\n", s ? s : "<UNDEFINED>");

	if (stored_bootdelay != -1 && s && !abortboot(stored_bootdelay)) {
		bool lock;
		int prev;

		lock = IS_ENABLED(CONFIG_AUTOBOOT_KEYED) &&
			!IS_ENABLED(CONFIG_AUTOBOOT_KEYED_CTRLC);
		if (lock)
			prev = disable_ctrlc(1); /* disable Ctrl-C checking */


		run_command_list(s, -1, 0);

		if (lock)
			disable_ctrlc(prev);	/* restore Ctrl-C checking */
	}

	if (IS_ENABLED(CONFIG_USE_AUTOBOOT_MENUKEY) &&
	    menukey == AUTOBOOT_MENUKEY) {
		s = env_get("menucmd");
		if (s)
			run_command_list(s, -1, 0);
	}
}

static int abortboot(int bootdelay)
{
	int abort = 0;
	
	return 0 ;   // 直接返回 
	
	if (bootdelay >= 0) {
		if (IS_ENABLED(CONFIG_AUTOBOOT_KEYED))
			abort = abortboot_key_sequence(bootdelay);
		else
			abort = abortboot_single_key(bootdelay);
	}


	if (IS_ENABLED(CONFIG_SILENT_CONSOLE) && abort)
		gd->flags &= ~GD_FLG_SILENT;

	return abort;
}

void autoboot_command(const char *s)
{
	debug("### main_loop: bootcmd=\"%s\"\n", s ? s : "<UNDEFINED>");

	if (stored_bootdelay != -1 && s && !abortboot(stored_bootdelay)) {
		bool lock;
		int prev;

		lock = IS_ENABLED(CONFIG_AUTOBOOT_KEYED) &&
			!IS_ENABLED(CONFIG_AUTOBOOT_KEYED_CTRLC);
		if (lock)
			prev = disable_ctrlc(1); /* disable Ctrl-C checking */


		run_command_list(s, -1, 0);  // 启动内核

		if (lock)
			disable_ctrlc(prev);	/* restore Ctrl-C checking */
	}

	if (IS_ENABLED(CONFIG_USE_AUTOBOOT_MENUKEY) &&
	    menukey == AUTOBOOT_MENUKEY) {
		s = env_get("menucmd");
		if (s)
			run_command_list(s, -1, 0);
	}
}

static int abortboot_single_key(int bootdelay)
{
	int abort = 0;
	unsigned long ts;

	printf("Hit any key to stop autoboot: %2d ", bootdelay);    // 打印

	/*
	 * Check if key already pressed
	 */
	if (tstc()) {	/* we got a key press	*/
		(void) getc();  /* consume input	*/
		puts("\b\b\b 0");
		abort = 1;	/* don't auto boot	*/
	}

	while ((bootdelay > 0) && (!abort)) {
		--bootdelay;
		/* delay 1000 ms */
		ts = get_timer(0);
		do {
			if (tstc()) {	/* we got a key press	*/
				int key;

				abort  = 1;	/* don't auto boot	*/
				bootdelay = 0;	/* no more delay	*/
				key = getc(); /* consume input	*/
				if (IS_ENABLED(CONFIG_USE_AUTOBOOT_MENUKEY))
					menukey = key;
				break;
			}
			udelay(10000);
		} while (!abort && get_timer(ts) < 1000);

		printf("\b\b\b%2d ", bootdelay);     // 打印倒计时  
	}

	putc('\n');

	return abort;
}