Linux的启动过程主要分三步，bootloader，kernel，roofts。
1 bootloader的文件系统配置和分区
bootloader一般使用uboot的比较多，这里以uboot举例。处理器要把uboot加入到内存中运行，这需要用到处理器内置的一部分程序。这个内置程序负责将这段uboot搬运到内存中。
1.1 bootloader在SD卡和emmc设备中
通过芯片的引脚配置可以决定程序存储在哪里，存储在sd卡和emmc中的时候呢，处理器内置程序一般会根据文件名来查找文件加载，比如BOOT.bin之类的名字，因此这个名字不能更改。文件系统一般使用fat32，因为兼容性比较好，这个也不能改。
就是内置程序会读取第一个分区中，名字为BOOT.bin，文件系统为fat32的文件进行加载。
1.2 bootloader在nor flash和nand flash设备中
在nor flash和nand flash中的时候一般内置程序会从地址0开始读取，读到有效的bin文件进行运行加载。因此此处不需要文件系统只需将uboot文件写入到flash的0地址处。
2 kernel的文件系统配置和分区
uboot运行之后会将kernel加载到内存中运行，因此这部分的配置取决于uboot的具体实现方法。
"norboot=echo Copying Linux from NOR flash to RAM... && " \"cp.b 0xE2100000 ${kernel_load_address} ${kernel_size} && " \
"cp.b 0xE2600000 ${devicetree_load_address} ${devicetree_size} && " \
"echo Copying ramdisk... && " \
"cp.b 0xE2620000 ${ramdisk_load_address} ${ramdisk_size} && " \
"bootm ${kernel_load_address} ${ramdisk_load_address} ${devicetree_load_address}\0" \
"qspiboot=echo Copying Linux from QSPI flash to RAM... && " \
"sf probe 0 0 0 && " \
"sf read ${kernel_load_address} 0x100000 ${kernel_size} && " \
"sf read ${devicetree_load_address} 0x600000 ${devicetree_size} && " \
"echo Copying ramdisk... && " \
"sf read ${ramdisk_load_address} 0x620000 ${ramdisk_size} && " \
"bootm ${kernel_load_address} ${ramdisk_load_address} ${devicetree_load_address}\0" \
"uenvboot=" \
"if run loadbootenv; then " \
"echo Loaded environment from ${bootenv}; " \
"run importbootenv; " \
"fi; " \
"if test -n $uenvcmd; then " \
"echo Running uenvcmd ...; " \
"run uenvcmd; " \
"fi\0" \
"sdboot=if mmcinfo; then " \
"run uenvboot; " \
"echo Copying Linux from SD to RAM... && " \
"load mmc 0 ${kernel_load_address} ${kernel_image} && " \
"load mmc 0 ${devicetree_load_address} ${devicetree_image} && " \
"load mmc 0 ${ramdisk_load_address} ${ramdisk_image} && " \
"bootm ${kernel_load_address} ${ramdisk_load_address} ${devicetree_load_address}; " \
"fi\0" \
"usbboot=if usb start; then " \
"run uenvboot; " \
"echo Copying Linux from USB to RAM... && " \
"load usb 0 ${kernel_load_address} ${kernel_image} && " \
"load usb 0 ${devicetree_load_address} ${devicetree_image} && " \
"load usb 0 ${ramdisk_load_address} ${ramdisk_image} && " \
"bootm ${kernel_load_address} ${ramdisk_load_address} ${devicetree_load_address}; " \
"fi\0" \
"nandboot=echo Copying Linux from NAND flash to RAM... && " \
"nand read ${kernel_load_address} 0x100000 ${kernel_size} && " \
"nand read ${devicetree_load_address} 0x600000 ${devicetree_size} && " \
"echo Copying ramdisk... && " \
"nand read ${ramdisk_load_address} 0x620000 ${ramdisk_size} && " \
"bootm ${kernel_load_address} ${ramdisk_load_address} ${devicetree_load_address}\0" \
"jtagboot=echo TFTPing Linux to RAM... && " \
"tftpboot ${kernel_load_address} ${kernel_image} && " \
"tftpboot ${devicetree_load_address} ${devicetree_image} && " \
"tftpboot ${ramdisk_load_address} ${ramdisk_image} && " \