auth: Allow to verify PublicKey

with platform format PK

In some platform the digest of the public key saved in the OTP is not
the the digest of the exact same public key buffer needed to check the
signature.  Typically, check signature may need a BER encapsulated
public key, but the hash saved in OTP may be the hash of the plain
public key.  Add a new platform weak function to transform the public
key buffer used by verify_signature to a buffer which hash is saved in
OTP.

Signed-off-by: Nicolas Toromanoff <nicolas.toromanoff@st.com>
Change-Id: I71017b41e3eca9398cededf317ad97e9b511be5f
Reviewed-on: https://gerrit.st.com/c/mpu/oe/st/tf-a/+/184836


Reviewed-by: CITOOLS <smet-aci-reviews@lists.codex.cro.st.com>
Reviewed-by: CIBUILD <smet-aci-builds@lists.codex.cro.st.com>
Reviewed-by: Yann GAUTIER <yann.gautier@st.com>