Add PKCS #5 v2.0 key derivation function 2 (PBKDF2)

This commit implements a crypto extension to support the key derivation
function defined in section 5.2 of RFC 2898
(https://www.ietf.org/rfc/rfc2898.txt), which is a re-publish of PKCS #5 v2.0.
The underlying pseudorandom function is HMAC-SHA1, which is the default PRF
specified in the RFC. It would be trivial to support the other HMAC functions
already implemented in OP-TEE.

See documentation/extensions/crypto_pbkdf2.md for details.

Tested on PLATFORM=vexpress-qemu_virt with the test vectors from RFC 6070
(https://www.ietf.org/rfc/rfc6070.txt).

Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>
Signed-off-by: Xiaoqiang Du <xiaoqiang.du@linaro.org>
"index" is a function in POSIX.1-2001 so it would cause compilation problems if a strings.h compliant with POSIX.1-2001 was ever included. It will probably never be a problem, but I never use that name to avoid having to consider whether it's a potential problem or not.