How to get android source code from github mirror

Get android source code using repo

Since the android.git.kernel.org is down now, so we can't get android source
code from there now. But AOSP has a mirror on github, so we can try to get
android source code there.

Android HAL

Android Hal 是用来隔离linux内核和Android用户空间的一个中间库，它主要是为了让一些硬件厂商不用根据linux内核的gpl协议放出全部的linux驱动代码。有了hal硬件厂商就可以只在内核驱动中实现基本的操作而把硬件操作的逻辑移到hal层来实现，hal层是apache协议的，所以厂商可以只提供二进制库。