[uboringssl] Align uboringssl with the //third_party/boringssl

This CL moves the assembly files and updates rules.mk to be more
consistent with //third_party/boringssl's layout. This allows the
scripts to check the subset and generate build files to be removed;
these are handled by roll_boringssl.go. The README has similarly been
simplified.

ZX-1991 #comment Adds the necessary changes

Change-Id: Ifc09dd27cd90c5c5381182fcc4f27e5400b97d05

[ulib][uboringssl] Use assembly rather than command line for noexecstack

The magic .note.GNU-stack dummy section has the same effect as the
assembler switch --noexecstack. However, the LTO compiler doesn't
like -Wa,--noexecstack on C files (since it never runs an assembler)
and complains.

Change-Id: I3e39d95de0854f6f3e94c2f618348c75e99515f2

[digest] Switch from cryptolib to uboringssl

This CL switches the SHA-256 implementation for ulib/digest to use
uboringssl, which includes optimized assembly and hardware acceleration
to provide better message digest performance.

Change-Id: Id3dedbc8232b4f4491b19fe46ef9e1415eea7358

[uboringssl] Add crypto support for AEADs

This CL expands the cryptographic routines supported by the Zircon port
of BoringSSL to include those needed for AEADs.

Change-Id: I34fe0122dbb9b8070184c5d98f706b23cf3586d5

[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

[uboringssl] Add crypto support for zxcrypt

This CL expands the cryptographic routines supported by the Zricon port
of BoringSSL to include those needed for zxcrypt.

Change-Id: I5c870fe002ac0c0af03e2c35d67d696aabc8825d

[zx] Magenta -> Zircon

The Great Renaming is here!

Change-Id: I3229bdeb2a3d0e40fb4db6fec8ca7d971fbffb94

[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