This is a clean-room implementation of the DRBG defined in SP800-90A.
All three viable DRBGs defined in the standard are implemented:

 * HMAC: This is the leanest DRBG and compiled per default
 * Hash: The more complex DRBG can be enabled at compile time
 * CTR: The most complex DRBG can also be enabled at compile time

The DRBG implementation offers the following:

 * All three DRBG types are implemented with a derivation function.
 * All DRBG types are available with and without prediction resistance.
 * All SHA types of SHA-1, SHA-256, SHA-384, SHA-512 are available for
   the HMAC and Hash DRBGs.
 * All AES types of AES-128, AES-192 and AES-256 are available for the
   CTR DRBG.
 * A self test is implemented with drbg_healthcheck().
 * The FIPS 140-2 continuous self test is implemented.

Signed-off-by: Stephan Mueller <smueller@chronox.de>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>