[rng] Choose HMAC_DRBG using SHA-256 as the DRBG algorithm

[rng] Choose HMAC_DRBG using SHA-256 as the DRBG algorithm

Both HMAC_DRBG using SHA-1 and HMAC_DRBG using SHA-256 are Approved
algorithms in ANS X9.82 for our chosen security strength of 128 bits.
However, general recommendations (see e.g. NIST SP800-57) are to use a
larger hash function in preference to SHA-1.

Since SHA-256 is required anyway for TLSv1.2 support, there is no code
size penalty for switching HMAC_DRBG to also use SHA-256.

Signed-off-by: Michael Brown <mcb30@ipxe.org>