x509: move common functions to x509 helper

Move x509_check_for_self_signed as a common helper function
that can be shared by legacy crypto lib and MbedTLS implementation.

Signed-off-by: Raymond Mao <raymond.mao@linaro.org>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
diff --git a/lib/crypto/x509_public_key.c b/lib/crypto/x509_public_key.c
index a10145a..4ba13c1 100644
--- a/lib/crypto/x509_public_key.c
+++ b/lib/crypto/x509_public_key.c
@@ -139,61 +139,7 @@
 	return ret;
 }
 
-/*
- * Check for self-signedness in an X.509 cert and if found, check the signature
- * immediately if we can.
- */
-int x509_check_for_self_signed(struct x509_certificate *cert)
-{
-	int ret = 0;
-
-	pr_devel("==>%s()\n", __func__);
-
-	if (cert->raw_subject_size != cert->raw_issuer_size ||
-	    memcmp(cert->raw_subject, cert->raw_issuer,
-		   cert->raw_issuer_size) != 0)
-		goto not_self_signed;
-
-	if (cert->sig->auth_ids[0] || cert->sig->auth_ids[1]) {
-		/* If the AKID is present it may have one or two parts.  If
-		 * both are supplied, both must match.
-		 */
-		bool a = asymmetric_key_id_same(cert->skid, cert->sig->auth_ids[1]);
-		bool b = asymmetric_key_id_same(cert->id, cert->sig->auth_ids[0]);
-
-		if (!a && !b)
-			goto not_self_signed;
-
-		ret = -EKEYREJECTED;
-		if (((a && !b) || (b && !a)) &&
-		    cert->sig->auth_ids[0] && cert->sig->auth_ids[1])
-			goto out;
-	}
-
-	ret = -EKEYREJECTED;
-	if (strcmp(cert->pub->pkey_algo, cert->sig->pkey_algo) != 0)
-		goto out;
-
-	ret = public_key_verify_signature(cert->pub, cert->sig);
-	if (ret < 0) {
-		if (ret == -ENOPKG) {
-			cert->unsupported_sig = true;
-			ret = 0;
-		}
-		goto out;
-	}
-
-	pr_devel("Cert Self-signature verified");
-	cert->self_signed = true;
-
-out:
-	pr_devel("<==%s() = %d\n", __func__, ret);
-	return ret;
-
-not_self_signed:
-	pr_devel("<==%s() = 0 [not]\n", __func__);
-	return 0;
-}
+#endif /* !CONFIG_IS_ENABLED(MBEDTLS_LIB_X509) */
 
 #ifndef __UBOOT__
 /*