[coreboot] Measuring the bootblock and adding a verstage

[Trammell Hudson]

I'd like to add a tlcl_measure() function to hash a region of code
and extend a PCR with the result.  I see that the Chromebook systems use
a verstage that links in src/lib/tlcl.c and there are sha1 functions in
3rdparty/chromeec/common/sha1.c, but neither of these are available from
the romstage on other boards.

For testing I've modified my romstage to include lib/tlcl.c and copied
sha1.c into lib.  This allows me to measure the bootblock and the romstage
from the romstage as soon as pch_enable_lpc() has been called, but it's
not clear to me how to enable verstage on other mainboards (like the
sandybridge in my x230).  Is there a guide or more documentation somewhere?