imx_51平台下，修改system/data分区的尺寸的步骤

1.For userdata.img, change the session value on the file vendor/fsl/imx51_BBG/BoardConfig.mk.

    add：TARGET_USERIMAGES_BLOCKS := 204800

2. For  system.img, do the followings:

    a. Add below line into vendor/fsl/imx51_BBG/BoardConfig.mk

  TARGET_SYSTEMIMAGES_BLOCKS := xxxx (block number you want for system partition. 1 block = 1KB, 204800 block=200MB)
   b. In build/core/Makefile, find the function define for "build-systemimage-target", and
       change
  $(call build-userimage-ext2-target, $(TARGET_OUT), $(1), system, true) --->>>
   $(call build-userimage-ext2-target,$(TARGET_OUT),$(1),system,true,$(TARGET_SYSTEMIMAGES_BLOCKS))
3.Remove the userdata.img and system.img under out/target/product/imx51_BBG/, then
   build again.