[crypto] Add HKDF class

This CL adds HKDF, a key derivation function class, as well as some small
helper files. ulib/digest will eventually need to be merged into
crypto::digest, but it is not necessary for this patch.

Change-Id: Iecaf7eb417d807baead7ffd912980b255ab816bc

[third_party] Roll BoringSSL

This CL updates the uboringssl subset of BoringSSL to match that in
//third_party/boringssl.

Change-Id: I37512741e9ec09b85a00811b5ec78ff9e7bd0a45

[third_party] Rename boring-crypto to uboringssl

After much effort, the BoringSSL team managed to get a subset of their
codebase through FIPS-140-2 certification. Unfortunately, they've named
the subset "BoringCrypto". To avoid the kind of confusion that agencies
that care about FIPS are particularly senstive to, this CL renames the
kernel's subset to uboringssl, pronounced 'micro-boringssl' with 'u' as
a stand-in for mu.

Change-Id: I4778a1847acc4451249f6f10f179c10bfb651454