Hi Jiongzhong,
You can go to i.MX 6 Series Software and Development Tool|NXP to download the BSP and documents you need.
Hope this can help you
Have a great day,
Dan
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repo init -u git://git.freescale.com/imx/fsl-arm-yocto-bsp.git -b imx-4.1.15-1.0.0_ga
repo sync
Thanks
Saurabh