xiaomi-santoni: use msm-3.18 upstream kernel
The current source being used for Xiaomi Redmi 4X (santoni) postmarketOS is from Xiaomi repo, which is pretty old 3.18.3x, and since we don't want outdated stuffs, I upstreamed the kernel with 3.18.136, which is from: https://github.com/android-linux-stable/msm-3.18
The kernel compiles, but does not boot with GCC 8, so GCC 6 is still being used.
I have tested the kernel, USB Networking, Display, USB OTG, Wi-Fi (with proprietary blobs) works fine, through I haven't figured out how to get Bluetooth and among of other stuffs to work. - Signed, Lancer.