[coreboot] hyper_threading in CMOS/NVRAM seems useless in many CPU models

[Iru Cai]

Hi,

I just tried to disable hyper-threading in my machine, and I found the
hyper_threading setting in NVRAM is useless. I searched in the code and
found only a few number of CPU code has in its Makefile.inc `subdirs-y +=
../hyperthreading', which can read the hyper_threading NVRAM option and
enable/disable hyper-threading.

Iru