Import Mediatek mips kernel configs

[sgalabov]

Summary

The idea is as follows:
There are 2 kernel config files - MEDIATEK_BASE and MEDIATEK.
MEDIATEK_BASE includes all essential definitions so a kernel can be built. It is not intended to be edited by the user.
MEDIATEK, which includes MEDIATEK_BASE, defines a default configuration (which happens to be for MT7620 SoC) and provides directions on what and how can be edited in order to support different cpu/SoC/driver set/dtb, etc.