mbedtls: add X509 cert parser porting layer

Add porting layer for X509 cert parser on top of MbedTLS X509
library.
Introduce _LEGACY and _MBEDTLS kconfigs for X509 cert parser legacy
and MbedTLS implementations respectively.

Signed-off-by: Raymond Mao <raymond.mao@linaro.org>
diff --git a/lib/mbedtls/Makefile b/lib/mbedtls/Makefile
index d3f566d..2965332 100644
--- a/lib/mbedtls/Makefile
+++ b/lib/mbedtls/Makefile
@@ -14,6 +14,8 @@
 # x509 libraries
 obj-$(CONFIG_$(SPL_)ASYMMETRIC_PUBLIC_KEY_MBEDTLS) += \
 	public_key.o
+obj-$(CONFIG_$(SPL_)X509_CERTIFICATE_PARSER_MBEDTLS) += \
+	x509_cert_parser.o
 
 # MbedTLS crypto library
 obj-$(CONFIG_MBEDTLS_LIB) += mbedtls_lib_crypto.o
@@ -44,7 +46,7 @@
 	$(MBEDTLS_LIB_DIR)/pk.o \
 	$(MBEDTLS_LIB_DIR)/pk_wrap.o \
 	$(MBEDTLS_LIB_DIR)/pkparse.o
-mbedtls_lib_x509-$(CONFIG_$(SPL_)X509_CERTIFICATE_PARSER) += \
+mbedtls_lib_x509-$(CONFIG_$(SPL_)X509_CERTIFICATE_PARSER_MBEDTLS) += \
 	$(MBEDTLS_LIB_DIR)/x509_crl.o \
 	$(MBEDTLS_LIB_DIR)/x509_crt.o
 mbedtls_lib_x509-$(CONFIG_$(SPL_)PKCS7_MESSAGE_PARSER) += \