Use a common KDF function

Cleanup the KDF function to use only one function per crypto backend.
Improve the KDF function to properly handle requested lenght and to
avoid unnecessarily reallocating buffers.

In OpenSSL use the new EVP_KDF API if available.

Signed-off-by: Simo Sorce <simo@redhat.com>
Reviewed-by: Anderson Toshiyuki Sasaki <ansasaki@redhat.com>
Reviewed-by: Andreas Schneider <asn@cryptomilk.org>