This adds the boringssl version of fiat-crypto. The version bundled here
had most if its non-used code removed, like signing, X25519 curve
operations not required by verification, etc.
Under boringssl tree, fiat can be found in third_party/fiat. The version
included here comes from a boringssl tree where the last commit is
f109f2087349712d3ac717d15fab48e130618110.
Signed-off-by: Fabio Utzig <utzig@apache.org>
Fabio Utzig