楼主，你好。
我现在也在做U盘升级的功能，
但是我现在卡在文件系统加载的过程了。
setenv loadaddr '0x12000000'
setenv initrd_addr '0x12C00000'
setenv initrd_high '0xffffffff'
setenv fdt_addr '0x18000000'
setenv load_file 'zImage_nor'
setenv fdt_file 'imx6dl-sabresd.dtb'
setenv ram_file 'fsl-image-mfgtool-initramfs-imx_mfgtools.cpio.gz.u-boot'
setenv bootcmd_update ' usb start; fatls usb 0:1; fatload usb 0:1 ${loadaddr} ${load_file};fatload usb 0:1 ${fdt_addr} ${fdt_file};fatload usb 0:1 ${initrd_addr} ${ram_file}; bootz ${loadaddr} - ${fdt_addr} - ${initrd_addr} '
setenv bootargs 'console=ttymxc0,115200 rdinit=/linuxrc video=mxcfb0:dev=ldb,bpp=32 video=mxcfb1:off video=mxcfb2:off video=mxcfb3:off vmalloc=256M'
setenv bootcmd 'run bootcmd_update'
saveenv
run bootcmd
之后kernel 出现错误：
另外通过对比mfgtoos烧录过程的日志发现，我自己编译的kernel在加载是少了两行关键的打印。
Trying to unpack rootfs image as initramfs...
Freeing initrd memory: 7304K (82c01000 - 83323000)
查找代码发现是在populate_rootfs（）函数中打印的。
但是这一段代码是在系统初始化自动加载的啊，不知为何会少了这两行打印。
run bootcmd
之后kernel 出现错误：
ALSA device list:
No soundcards found.
VFS: Cannot open root device "(null)" or unknown-block(0,0): error -6
Please append a correct "root=" boot option; here are the available partitions:
1f00 768 mtdblock0 (driver?)
1f01 128 mtdblock1 (driver?)
1f02 7296 mtdblock2 (driver?)
b300 15267840 mmcblk3 driver: mmcblk
b301 16383 mmcblk3p1 00000000-01
b302 16384 mmcblk3p2 00000000-02
b303 1 mmcblk3p3
b304 3643392 mmcblk3p4 00000000-04
b305 524287 mmcblk3p5 00000000-05
b306 524287 mmcblk3p6 00000000-06
b307 8191 mmcblk3p7 00000000-07
b308 8191 mmcblk3p8 00000000-08
b309 10485759 mmcblk3p9 00000000-09
b330 4096 mmcblk3rpmb (driver?)
b320 4096 mmcblk3boot1 (driver?)
b310 4096 mmcblk3boot0 (driver?)
Kernel panic - not syncing: VFS: Unable to mount root fs on unknown-block(0,0)
CPU1: stopping
CPU: 1 PID: 0 Comm: swapper/1 Not tainted 3.14.52+ #1
[<800150ec>] (unwind_backtrace) from [<80011c08>] (show_stack+0x10/0x14)
[<80011c08>] (show_stack) from [<8065c4cc>] (dump_stack+0x7c/0xbc)
[<8065c4cc>] (dump_stack) from [<80013ff8>] (handle_IPI+0x1a8/0x1c0)
[<80013ff8>] (handle_IPI) from [<800085c4>] (gic_handle_irq+0x58/0x5c)
[<800085c4>] (gic_handle_irq) from [<80012700>] (__irq_svc+0x40/0x70)
Exception stack(0xd60a3f50 to 0xd60a3f98)
3f40: d60a3f98 00000001 8818eea4 00000000
3f60: 83f0b2a2 00000000 eef36110 809d65d4 00000001 eef36114 d60a2000 d60a2000
3f80: 00000017 d60a3f98 80078b04 80472174 60000113 ffffffff
[<80012700>] (__irq_svc) from [<80472174>] (cpuidle_enter_state+0x50/0xe0)
[<80472174>] (cpuidle_enter_state) from [<80472300>] (cpuidle_idle_call+0xfc/0x150)
[<80472300>] (cpuidle_idle_call) from [<8000f10c>] (arch_cpu_idle+0x8/0x44)
[<8000f10c>] (arch_cpu_idle) from [<8006d5b8>] (cpu_startup_entry+0x104/0x150)
[<8006d5b8>] (cpu_startup_entry) from [<10008664>] (0x10008664)
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