util: Add me_cleaner

me_cleaner is a tool to strip down Intel ME/TXE images by removing all
the non-fundamental code, while keeping the ME/TXE image valid and
suitable for booting the system. The remaining code (ROMP and BUP
modules) is the one responsible for the very basic initialization of
the ME/TXE subsystem and can't be removed.

This tool exploits the fact that:
 * Each ME/TXE partition is signed individually and it is possible to
    remove both the partition and the signature.
 * The ME/TXE modules are not signed directly, instead they are hashed
    and the list of their hashes is hashed again and signed: this
    means that modifying a module doesn't invalidate the signature,
    but only the hash of that single module.
 * The modules hashes are checked only when the corresponding module
    needs to be executed.
 * The system can boot after the execution of the first module (BUP,
    inside the FTPR partition), even if the subsequent stages fail.

Currently me_cleaner works on every Intel platform with Intel ME or
Intel TXE with the following limitations:
 * Doesn't work when Intel Boot Guard is set in Verified Boot mode.
 * Doesn't fully work on Nehalem yet.
 * On Skylake and later generations, since the partitions' internal
    structure has changed, me_cleaner leaves intact the FTPR
    partition, removing all the the other partitions.

This tool has been tested on multiple platforms and architectures by
different users, and seems to be stable. The reports are available
here:
https://github.com/corna/me_cleaner/issues/3

A more in-depth description of me_cleaner is available here:
https://github.com/corna/me_cleaner/wiki/How-does-it-work%3F

Change-Id: I9013799e9adea0dea0775b9afe718de5fc4ca748
Signed-off-by: Nicola Corna <nicola@corna.info>
Reviewed-on: https://review.coreboot.org/18203
Tested-by: build bot (Jenkins)
Reviewed-by: Philipp Deppenwiese <zaolin.daisuki@gmail.com>
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>

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