U-Boot第一阶段的启动流程。(nandflash启动，把nand的4k代码考到sram中，因为nand没址线，不能映射到内存，所以通过sram进行过度，sram中4k代码把整个uboot拷贝到sdram上，初始化好堆栈，为c语言提供条件，进入uboot的第二阶段！ )这个阶段主要是初始化硬件设备，为加载U-Boot的第二阶段代码准备RAM空间最后跳转到lib_arm/board.c中start_armboot函数，这是第二阶段的入口点。(U-Boot first phase of the startup process. (second stage nandflash start, NAND 4K code to test SRAM, because NAND did not address line, cannot be mapped into memory, so through the SRAM 4K code in SRAM over the whole uboot copy to the SDRAM, a good initialization stack, provide the conditions for the C language into uboot! This stage is mainly to initialize hardware devices, to prepare RAM space for loading the U-Boot's second stage code, and finally jump to start_armboot function in lib_arm/board.c, which is the entry point of the second stage.)