在启动代码start.S中，代码的流程是什么样的呢？

　　(reset) <arch/arm/cpu/armv7/start.S > (b lowlevel_init: arch/arm/cpu/armv7/lowlevel_init.S)
 (b _main) --> <arch/arm/lib/crt0.S> (bl board_init_f) --> <arch/arm/lib/spl.c> (board_init_r)
--> <common/spl/spl.c> (jump_to_image_no_args去启动u-boot)