we have these folders in this repo:

prebuild:
	to store header files and libraries for cross compling.
	target:
		file to build up a root filesystem. there are two subfolders:
		1. common: for platform independent files
		2. overlay: for platform dependent files
		when building rootfs, files in overlay folder will be merged
		into common folder.


tools:
	bin and scritps for generating rootfs

configs:
	we have two types of configuration file:
	1. txt: for file list in ramdisk
	2. its: for generating itb file