[coreboot] x86 automatic ROM caching enabled

[Aaron Durbin]

Hi folks,

I uploaded http://review.coreboot.org/#/c/3004/ to do an auto
enablement of caching the ROM. I put some comments on the patch.
Basically, if the cpu is a hyperthreaded CPU and all the APs run
through the MTRR code the auto caching of the ROM will be dropped
because the MTRR MSRs are core scoped -- not thread scope. I am of the
opinion I should drop that change and just fix up the users (only
ChromeOS devices on a single chipset at the moment) that really desire
this behavior by calling x86_mtrr_enable_rom_caching() at the
appropriate point in the code. In addition I plan on putting a comment
in mtrr.h to warn people of the side effects.

I just wanted to hear what others' opinions were.

Thanks for the feedback.

-Aaron