cert_tool: Update for ecdsa brainpoolP256t1 support

Updated cert_tool to be able to select brainpool P256t1
or NIST prim256v1 curve for certificates signature.

Signed-off-by: Nicolas Toromanoff <nicolas.toromanoff@st.com>
Change-Id: I6e800144697069ea83660053b8ba6e21c229243a
Reviewed-on: https://gerrit.st.com/c/mpu/oe/st/tf-a/+/184835


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>