Import Mediatek/Ralink kernel configs

[sgalabov]

Summary

In order to build a kernel with one of these configs the user should do the following:

1. Give the appropriate board dts file to be used by either:

1.1. edit the SoC kernel config required (e.g., MT7620A_FDT) and include the required FDT_DTS_FILE makeoption; or
1.2. simply supply FDT_DTS_FILE="xx.dts" on the command line when building the kernel
Of course, the user can also create a completely new kernel config to match the desired board and include the SoC kernel config from within it.

2. If required, edit the MEDIATEK config file, which includes optional drivers and comment out the unneeded ones.

2.1. this would only make sense if kernel size is a concern. Even if we build the kernel with all drivers, if we lzma it and package it as a uImage, its size is still around 1.1MiB.

The user will have to choose a dts file (or create a new one) from sys/gnu/dts/mips , where all Mediatek/Ralink dts files will be imported via a later revision.