http://blog.youkuaiyun.com/kaiwii/article/details/7681801
another example:
Unzip ramdisk:
gunzip -c ../ramdisk.img |cpio –i
“modifyinit script”
find . | cpio -o -H newc |gzip > ../newramdisk.img
mkbootimg --kernel ./uImage--ramdisk ./ramdisk.img -o boot.img