Adds the support of key derivation using the scheme hkdf.
This scheme is defined in rfc5869.

Signed-off-by: Philippe Reynes <philippe.reynes@softathome.com>
Reviewed-by: Raymond Mao <raymond.mao@linaro.org>

Adds the support of the hmac based on sha256.
This implementation is based on rfc2104.

Signed-off-by: Philippe Reynes <philippe.reynes@softathome.com>
Reviewed-by: Raymond Mao <raymond.mao@linaro.org>

The function sha256_csum_wd is defined in lib/sha256.c
and in lib/mbedtls/sha256.c. To avoid duplicating this
function (and future function), we move this function
to the file lib/sha256_common.c

Reviewed-by: Raymond Mao <raymond.mao@linaro.org>
Signed-off-by: Philippe Reynes <philippe.reynes@softathome.com>

Implement digest shim layer on top of MbedTLS crypto library.
Introduce <alg>_MBEDTLS kconfig for MbedTLS crypto implementations.

Signed-off-by: Raymond Mao <raymond.mao@linaro.org>