The code source is separated to three parts, AOSP & Kernel & SLA, You can download them one by one, and merge them into a whole one.

1、AOSP

repo init -u https://android.googlesource.com/platform/manifest -b
android-6.0.0_r1
Repo sync –j16

2、Linux

https://github.com/helio-x20/linux/tree/helio-x20

3、SLA

https://builds.96boards.org/releases/helio-x20/mediatek/aosp/latest/

You can get a whole set of source code after following below steps.

1、repo get the aosp code source
2、Move the linux code source to AOSP, and rename it to kernel-3.18
3、Unzip the SLA tar ball, cover the appropriate files.