[coreboot] Shrinking the ME region to make more space for CBFS

[Nathaniel Roach]

Hi all!
It was suggested to me that some of you on the list might be interested 
so I'm chucking it up here.

Once you run me_cleaner in it's current revision, it's possible to trim 
the ME region file down to the last relevant (read: non 0xFF) byte and 
then shrink the region in the IFD - freeing up to about 4MiB for use in 
CBFS or for other things. (This means you should be able to fit a linux 
kernel in flash now too)

I'd initially written a rough how to here: 
https://nroach44.id.au/index.php/2017/01/24/thinkpad-x220-shrink-the-me-region/ 
but since I've done some more work & some tweaks, I'm planning to 
re-write it when me_cleaner integrates the "minimum size" readout that 
makes this a bit easier.

I've tested this with my X220 (which had the full 5MiB region) and, as 
per comments on the me_cleaner github, the 1.5MiB image from the 
coreboot wiki. The former shrinks to ~900K while the latter shrinks to 
~300K, and there's no adverse effects to the operation of the machine.

It's been tested on my X220 and github user alegru has tested it on 
their T420, so I'm not sure if it will work for anything !(SANDYBRIDGE).