Add export_public_key entry point to p256-m driver

Signed-off-by: Manuel Pégourié-Gonnard <manuel.pegourie-gonnard@arm.com>
diff --git a/3rdparty/p256-m/p256-m_driver_entrypoints.c b/3rdparty/p256-m/p256-m_driver_entrypoints.c
index e48b4e8..6131dae 100644
--- a/3rdparty/p256-m/p256-m_driver_entrypoints.c
+++ b/3rdparty/p256-m/p256-m_driver_entrypoints.c
@@ -104,6 +104,40 @@
     return PSA_SUCCESS;
 }
 
+psa_status_t p256_transparent_export_public_key(const psa_key_attributes_t *attributes,
+                                    const uint8_t *key_buffer,
+                                    size_t key_buffer_size,
+                                    uint8_t *data,
+                                    size_t data_size,
+                                    size_t *data_length)
+{
+    /* Is this the right curve? */
+    size_t bits = psa_get_key_bits(attributes);
+    psa_key_type_t type = psa_get_key_type(attributes);
+    if (bits != 256 || type != PSA_KEY_TYPE_ECC_KEY_PAIR(PSA_ECC_FAMILY_SECP_R1)) {
+        return PSA_ERROR_NOT_SUPPORTED;
+    }
+
+    /* Validate input and output sizes */
+    if (key_buffer_size != 32) {
+        return PSA_ERROR_INVALID_ARGUMENT;
+    }
+    if (data_size < 65) {
+        return PSA_ERROR_BUFFER_TOO_SMALL;
+    }
+
+    /* Output public key in the PSA export format */
+    data[0] = 0x04;
+    int ret = p256_public_from_private(data + 1, key_buffer);
+    if (ret != P256_SUCCESS) {
+        /* The only possible error is the private key was invalid */
+        return PSA_ERROR_INVALID_ARGUMENT;
+    }
+    *data_length = 65;
+
+    return PSA_SUCCESS;
+}
+
 psa_status_t p256_transparent_generate_key(
     const psa_key_attributes_t *attributes,
     uint8_t *key_buffer,