[fdio] Move headers into lib/fdio/...

Change-Id: Ie8d74e716da913bf6e2672c4acf8cd67b4962b7f

[crypto][fdio][zxcrypt] Improve debug output

This CL improves fdio/debug's xprintf with location info.

Change-Id: Idd05c0439258ed3fe160086bcce05575eddbc38b

[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