[coreboot-gerrit] Change in coreboot[master]: intel/stm: Add platform opt-in Kconfig

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Change subject: intel/stm: Add platform opt-in Kconfig
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Patch Set 1:

Need to also add this to the other Intel processor families Kconfig's as well.  Generally, if the processor supports VTX it supports the STM. My oldest platform running an STM is a Sandybridge.  And we were working on the family proceeding Sandybridge as well.

Also, I will also all an additional feature MSR (in stm_setup) check that indicates if that particular processor supports an STM.  In any case, if the STM setup is not successful, IA32_SMM_MONITOR_VALID does not get set in the IA32_SMM_MONITOR_CTL_MSR.