Add per-soc options to allow custom kernels.

[andrew]

Summary

This will allow us to have custom cut down kernels supporting just
a single SoC.

[andrew]

sys/conf/options.arm64

10 ↗

(On Diff #8196)

I'm planning on adding an option for the HiSilicon hi6220 in the HiKey board to enable the custom drivers we will need there.

[zbb]

So when we have a networking card driver for Thunder and would like to disable it in the kernel configuration, we will need to disable all ThunderX drivers?

[andrew]

No more than you would any other device, you would define it in files.arm64 something like:

arm64/cavium/thunder_nic.c optional soc_cavm_thunderx thndr_nic

I moved the PCIe driver as I did because it's unlikely someone would be interested in pci on one SoC, but not on another.

[imp]

I like this.